Promote better deprecation communication and compatibility

src/stylesheets/core/_deprecated.scss

@@ -0,0 +1,52 @@
+/*  deprecated.scss
+    ---
+    Occasionally the design system will deprecate
+    old variables or functionality. If we replace
+    the old functionality with something new, this is a
+    place to connect the old functionality to the
+    new functionality, in the service of better
+    continuity and backwards compatibility within a
+    major release cycle.
+
+    Note the USWDS version where we deprecated the
+    old functionality in a comment.
+
+    Add the deprecation to the warning.
+
+    This file should started fresh at each
+    major version.
+*/
+
+@if $theme-show-deprecation-warnings {
+  @warn "\a"
+    + "--------------------------------------------------------------------\a"
+    + "Deprecated in USWDS 2.x\a"
+    + "--------------------------------------------------------------------\a"
+    + "2.2.0\a"
+    + "$theme-navigation-width \2192  $theme-header-min-width\a"
+    + "$theme-megamenu-logo-text-width \2192  $theme-header-logo-text-width\a"
+    + "--------------------------------------------------------------------\a"
+    + "2.0.2\a"
+    + "$theme-title-font-size \2192  $theme-display-font-size\a"
+    + "@include title \2192  @include display\a"
+    + "@include typeset-title \2192  @include typeset-display\a"
+    + "--------------------------------------------------------------------\a"
+    + "Disable this message using \a"
+    + "$theme-show-deprecation-warnings: false\a"
+    + "--------------------------------------------------------------------\a";
+}
+
+// Deprecated in 2.2.0
+$theme-navigation-width: $theme-header-min-width;
+$theme-megamenu-logo-text-width: $theme-header-logo-text-width;
+
+// Deprecated in 2.0.2
+$theme-title-font-size: $theme-display-font-size;
+
+@mixin title {
+  @include display;
+}
+
+@mixin typeset-title {
+  @include typeset-display;
+}

src/stylesheets/packages/_required.scss

@@ -15,3 +15,6 @@
 @import "../core/properties";
 @import "../core/mixins/all";
 @import "../core/placeholders/all";
+
+// deprecated import needs to be last
+@import "../core/deprecated";

src/stylesheets/settings/_settings-general.scss

@@ -33,10 +33,13 @@ Show compile warnings
 ----------------------------------------
 Show Sass warnings when functions and
 mixins use non-standard tokens.
+AND
+Show deprecation notifications.
 ----------------------------------------
 */
 
 $theme-show-compile-warnings: true !default;
+$theme-show-deprecation-warnings: true !default;
 
 /*
 ----------------------------------------

src/stylesheets/theme/_uswds-theme-general.scss

@@ -33,10 +33,13 @@ Show compile warnings
 ----------------------------------------
 Show Sass warnings when functions and
 mixins use non-standard tokens.
+AND
+Show deprecation notifications.
 ----------------------------------------
 */
 
 $theme-show-compile-warnings: true;
+$theme-show-deprecation-warnings: true;
 
 /*
 ----------------------------------------