diff --git a/packages/ember-template-compiler/index.ts b/packages/ember-template-compiler/index.ts
index 85baf317ec0..e65ee16e7de 100644
--- a/packages/ember-template-compiler/index.ts
+++ b/packages/ember-template-compiler/index.ts
@@ -31,8 +31,6 @@ export {
   default as compileOptions,
   buildCompileOptions as _buildCompileOptions,
   transformsFor as _transformsFor,
-  registerPlugin,
-  unregisterPlugin,
 } from './lib/system/compile-options';
 export { RESOLUTION_MODE_TRANSFORMS, STRICT_MODE_TRANSFORMS } from './lib/plugins/index';
 export { EmberPrecompileOptions } from './lib/types';
diff --git a/packages/ember-template-compiler/lib/system/compile-options.ts b/packages/ember-template-compiler/lib/system/compile-options.ts
index f6d42c8b2ff..37f660d3dfe 100644
--- a/packages/ember-template-compiler/lib/system/compile-options.ts
+++ b/packages/ember-template-compiler/lib/system/compile-options.ts
@@ -1,6 +1,5 @@
 import { EMBER_STRICT_MODE } from '@ember/canary-features';
-import { assert, deprecate } from '@ember/debug';
-import { AST, ASTPlugin, ASTPluginEnvironment, Syntax } from '@glimmer/syntax';
+import { assert } from '@ember/debug';
 import { RESOLUTION_MODE_TRANSFORMS, STRICT_MODE_TRANSFORMS } from '../plugins/index';
 import { EmberPrecompileOptions, PluginFunc } from '../types';
 import COMPONENT_NAME_SIMPLE_DASHERIZE_CACHE from './dasherize-component-name';
@@ -63,127 +62,12 @@ export default function compileOptions(
   } else {
     let potententialPugins = [...USER_PLUGINS, ...builtInPlugins];
     assert('expected plugins', options.plugins);
-    let providedPlugins = options.plugins.ast.map((plugin) => wrapLegacyPluginIfNeeded(plugin));
     let pluginsToAdd = potententialPugins.filter((plugin) => {
       assert('expected plugins', options.plugins);
       return options.plugins.ast.indexOf(plugin) === -1;
     });
-    options.plugins.ast = providedPlugins.concat(pluginsToAdd);
+    options.plugins.ast = options.plugins.ast.concat(pluginsToAdd);
   }
 
   return options;
 }
-
-interface LegacyPlugin {
-  transform(node: AST.Program): AST.Node;
-  syntax: Syntax;
-}
-
-export type LegacyPluginClass = new (env: ASTPluginEnvironment) => LegacyPlugin;
-
-function isLegacyPluginClass(plugin: PluginFunc | LegacyPluginClass): plugin is LegacyPluginClass {
-  return plugin.prototype && typeof plugin.prototype.transform === 'function';
-}
-
-function wrapLegacyPluginIfNeeded(plugin: PluginFunc | LegacyPluginClass): PluginFunc {
-  if (isLegacyPluginClass(plugin)) {
-    const Plugin = plugin;
-
-    deprecate(
-      `Using class based template compilation plugins is deprecated, please update to the functional style: ${Plugin.name}`,
-      false,
-      {
-        id: 'template-compiler.registerPlugin',
-        until: '4.0.0',
-        for: 'ember-source',
-        since: {
-          enabled: '3.27.0',
-        },
-      }
-    );
-
-    const pluginFunc: PluginFunc = (env: ASTPluginEnvironment): ASTPlugin => {
-      let pluginInstantiated = false;
-
-      return {
-        name: plugin.name,
-
-        visitor: {
-          Program(node: AST.Program): AST.Node | void {
-            if (!pluginInstantiated) {
-              pluginInstantiated = true;
-              let instance = new Plugin(env);
-
-              instance.syntax = env.syntax;
-
-              return instance.transform(node);
-            }
-          },
-        },
-      };
-    };
-
-    pluginFunc.__raw = Plugin;
-
-    return pluginFunc;
-  } else {
-    return plugin;
-  }
-}
-
-export function registerPlugin(type: string, _plugin: PluginFunc | LegacyPluginClass): void {
-  deprecate(
-    'registerPlugin is deprecated, please pass plugins directly via `compile` and/or `precompile`.',
-    false,
-    {
-      id: 'template-compiler.registerPlugin',
-      until: '4.0.0',
-      for: 'ember-source',
-      since: {
-        enabled: '3.27.0',
-      },
-    }
-  );
-
-  if (type !== 'ast') {
-    throw new Error(
-      `Attempting to register ${_plugin} as "${type}" which is not a valid Glimmer plugin type.`
-    );
-  }
-
-  for (let i = 0; i < USER_PLUGINS.length; i++) {
-    let PLUGIN = USER_PLUGINS[i];
-    if (PLUGIN === _plugin || PLUGIN.__raw === _plugin) {
-      return;
-    }
-  }
-
-  let plugin = wrapLegacyPluginIfNeeded(_plugin);
-
-  USER_PLUGINS = [plugin, ...USER_PLUGINS];
-}
-
-export function unregisterPlugin(type: string, PluginClass: PluginFunc | LegacyPluginClass): void {
-  deprecate(
-    'unregisterPlugin is deprecated, please pass plugins directly via `compile` and/or `precompile`.',
-    false,
-    {
-      id: 'template-compiler.registerPlugin',
-      until: '4.0.0',
-      for: 'ember-source',
-      since: {
-        enabled: '3.27.0',
-      },
-    }
-  );
-
-  if (type !== 'ast') {
-    throw new Error(
-      `Attempting to unregister ${PluginClass} as "${type}" which is not a valid Glimmer plugin type.`
-    );
-  }
-
-  USER_PLUGINS = USER_PLUGINS.filter(
-    (plugin) => plugin !== PluginClass && plugin.__raw !== PluginClass
-  );
-}
diff --git a/packages/ember-template-compiler/lib/types.d.ts b/packages/ember-template-compiler/lib/types.d.ts
index 35cc1c710b1..7b2b85eb346 100644
--- a/packages/ember-template-compiler/lib/types.d.ts
+++ b/packages/ember-template-compiler/lib/types.d.ts
@@ -4,13 +4,16 @@ import {
   builders,
   PrecompileOptions,
 } from '@glimmer/syntax';
-import { LegacyPluginClass } from './system/compile-options';
 
 export type Builders = typeof builders;
 
-export interface PluginFunc extends ASTPluginBuilder<EmberASTPluginEnvironment> {
-  __raw?: LegacyPluginClass;
-}
+/*
+ * It seems like it should be possible to reepxport the `ASTPluginBuilder`
+ * interface with a new named export, but the I wasn't able to figure out the
+ * typing. Here export the interface subclass with no modification.
+ */
+// eslint-disable-next-line @typescript-eslint/no-empty-interface
+export interface PluginFunc extends ASTPluginBuilder<EmberASTPluginEnvironment> {}
 
 interface Plugins {
   ast: PluginFunc[];
diff --git a/packages/ember-template-compiler/tests/basic-usage-test.js b/packages/ember-template-compiler/tests/basic-usage-test.js
index 4626b599048..9889f8c34f9 100644
--- a/packages/ember-template-compiler/tests/basic-usage-test.js
+++ b/packages/ember-template-compiler/tests/basic-usage-test.js
@@ -1,23 +1,6 @@
-import {
-  _buildCompileOptions,
-  _preprocess,
-  _print,
-  registerPlugin,
-  unregisterPlugin,
-} from '../index';
+import { _buildCompileOptions, _preprocess, _print } from '../index';
 import { moduleFor, RenderingTestCase } from 'internal-test-helpers';
 
-function reverseElementNodeTag() {
-  return {
-    name: 'reverse-element-node-tag',
-    visitor: {
-      ElementNode(node) {
-        node.tag = node.tag.split('').reverse().join('');
-      },
-    },
-  };
-}
-
 function removeDataTest() {
   return {
     name: 'remove-data-test',
@@ -39,13 +22,6 @@ function removeDataTest() {
 moduleFor(
   'ember-template-compiler: Embroider-like compilation',
   class extends RenderingTestCase {
-    afterEach() {
-      expectDeprecation(() => {
-        unregisterPlugin('ast', removeDataTest);
-      }, /unregisterPlugin is deprecated, please pass plugins directly via `compile` and\/or `precompile`/);
-      return super.afterEach();
-    }
-
     '@test can process a subset of AST plugins and print'(assert) {
       let template = '<div data-test="foo" data-blah="derp" class="hahaha">&nbsp;</div>';
 
@@ -63,30 +39,5 @@ moduleFor(
 
       assert.equal(result, '<div data-blah="derp" class="hahaha">&nbsp;</div>');
     }
-
-    '@test registerPlugin based transforms can be avoided'(assert) {
-      expectDeprecation(() => {
-        registerPlugin('ast', removeDataTest);
-      }, /registerPlugin is deprecated, please pass plugins directly via `compile` and\/or `precompile`/);
-
-      let template = '<div data-test="foo" data-blah="derp" class="hahaha">&nbsp;</div>';
-
-      // build up options including strictMode default values, customizeComponentName, meta.moduleName, etc
-      let options = _buildCompileOptions({
-        mode: 'codemod',
-        moduleName: 'components/foo',
-        plugins: {
-          ast: [reverseElementNodeTag],
-        },
-      });
-
-      let transformedTemplateAST = _preprocess(template, options);
-
-      // print back to a handlebars string
-      let result = _print(transformedTemplateAST, { entityEncoding: 'raw' });
-
-      // only reverseElementNodeTag has ran, **not** removeDataTest
-      assert.equal(result, '<vid data-test="foo" data-blah="derp" class="hahaha">&nbsp;</vid>');
-    }
   }
 );
diff --git a/packages/ember-template-compiler/tests/system/compile_options_test.js b/packages/ember-template-compiler/tests/system/compile_options_test.js
index 2b75b69f36d..36e4afb82a9 100644
--- a/packages/ember-template-compiler/tests/system/compile_options_test.js
+++ b/packages/ember-template-compiler/tests/system/compile_options_test.js
@@ -3,8 +3,6 @@ import {
   compileOptions,
   RESOLUTION_MODE_TRANSFORMS,
   STRICT_MODE_TRANSFORMS,
-  registerPlugin,
-  unregisterPlugin,
 } from '../../index';
 import { moduleFor, AbstractTestCase, RenderingTestCase } from 'internal-test-helpers';
 
@@ -49,38 +47,7 @@ moduleFor(
   }
 );
 
-let customTransformCounter = 0;
-class LegacyCustomTransform {
-  constructor(options) {
-    customTransformCounter++;
-    this.options = options;
-    this.syntax = null;
-  }
-
-  transform(ast) {
-    let walker = new this.syntax.Walker();
-
-    walker.visit(ast, (node) => {
-      if (node.type !== 'ElementNode') {
-        return;
-      }
-
-      for (let i = 0; i < node.attributes.length; i++) {
-        let attribute = node.attributes[i];
-
-        if (attribute.name === 'data-test') {
-          node.attributes.splice(i, 1);
-        }
-      }
-    });
-
-    return ast;
-  }
-}
-
 function customTransform() {
-  customTransformCounter++;
-
   return {
     name: 'remove-data-test',
 
@@ -98,113 +65,6 @@ function customTransform() {
   };
 }
 
-class CustomPluginsTests extends RenderingTestCase {
-  afterEach() {
-    customTransformCounter = 0;
-    return super.afterEach();
-  }
-
-  ['@test custom plugins can be used']() {
-    this.render('<div data-test="foo" data-blah="derp" class="hahaha"></div>');
-    this.assertElement(this.firstChild, {
-      tagName: 'div',
-      attrs: { class: 'hahaha', 'data-blah': 'derp' },
-      content: '',
-    });
-  }
-
-  ['@test wrapped plugins are only invoked once per template'](assert) {
-    this.render('<div>{{#if this.falsey}}nope{{/if}}</div>');
-    assert.equal(customTransformCounter, 1, 'transform should only be instantiated once');
-  }
-}
-
-moduleFor(
-  'ember-template-compiler: [DEPRECATED] registerPlugin with a custom plugins in legacy format',
-  class extends CustomPluginsTests {
-    beforeEach() {
-      expectDeprecation(
-        `Using class based template compilation plugins is deprecated, please update to the functional style: ${LegacyCustomTransform.name}`
-      );
-      expectDeprecation(
-        'registerPlugin is deprecated, please pass plugins directly via `compile` and/or `precompile`.'
-      );
-      registerPlugin('ast', LegacyCustomTransform);
-    }
-
-    afterEach() {
-      expectDeprecation(() => {
-        unregisterPlugin('ast', LegacyCustomTransform);
-      }, /unregisterPlugin is deprecated, please pass plugins directly via `compile` and\/or `precompile`/);
-      return super.afterEach();
-    }
-
-    ['@test custom registered plugins are deduplicated'](assert) {
-      expectDeprecation(
-        `Using class based template compilation plugins is deprecated, please update to the functional style: ${LegacyCustomTransform.name}`
-      );
-      expectDeprecation(
-        'registerPlugin is deprecated, please pass plugins directly via `compile` and/or `precompile`.'
-      );
-      registerPlugin('ast', LegacyCustomTransform);
-
-      this.registerTemplate(
-        'application',
-        '<div data-test="foo" data-blah="derp" class="hahaha"></div>'
-      );
-      assert.equal(customTransformCounter, 1, 'transform should only be instantiated once');
-    }
-  }
-);
-
-moduleFor(
-  'ember-template-compiler: [DEPRECATED] registerPlugin with a custom plugins',
-  class extends CustomPluginsTests {
-    beforeEach() {
-      expectDeprecation(() => {
-        registerPlugin('ast', customTransform);
-      }, /registerPlugin is deprecated, please pass plugins directly via `compile` and\/or `precompile`/);
-    }
-
-    afterEach() {
-      expectDeprecation(() => {
-        unregisterPlugin('ast', customTransform);
-      }, /unregisterPlugin is deprecated, please pass plugins directly via `compile` and\/or `precompile`/);
-
-      return super.afterEach();
-    }
-
-    ['@test custom registered plugins are deduplicated'](assert) {
-      expectDeprecation(() => {
-        registerPlugin('ast', customTransform);
-      }, /registerPlugin is deprecated, please pass plugins directly via `compile` and\/or `precompile`/);
-
-      this.registerTemplate(
-        'application',
-        '<div data-test="foo" data-blah="derp" class="hahaha"></div>'
-      );
-      assert.equal(customTransformCounter, 1, 'transform should only be instantiated once');
-    }
-  }
-);
-
-moduleFor(
-  'ember-template-compiler: custom plugins in legacy format passed to compile',
-  class extends RenderingTestCase {
-    // override so that we can provide custom AST plugins to compile
-    compile(templateString) {
-      expectDeprecation(
-        'Using class based template compilation plugins is deprecated, please update to the functional style: LegacyCustomTransform'
-      );
-      return compile(templateString, {
-        plugins: {
-          ast: [LegacyCustomTransform],
-        },
-      });
-    }
-  }
-);
-
 moduleFor(
   'ember-template-compiler: custom plugins passed to compile',
   class extends RenderingTestCase {
diff --git a/packages/ember/index.js b/packages/ember/index.js
index ffa9aeeb8b3..e1750535291 100644
--- a/packages/ember/index.js
+++ b/packages/ember/index.js
@@ -603,7 +603,6 @@ function defineEmberTemplateCompilerLazyLoad(key) {
 
         EmberHTMLBars.precompile = EmberHandlebars.precompile = templateCompiler.precompile;
         EmberHTMLBars.compile = EmberHandlebars.compile = templateCompiler.compile;
-        EmberHTMLBars.registerPlugin = templateCompiler.registerPlugin;
 
         Object.defineProperty(Ember, 'HTMLBars', {
           configurable: true,