chore: deprecate MapTo variants

Deprecating MapTo variants, as they were only wrappers around the Map variants, and added unnecessary API surface area.

related ReactiveX#6367
resolves ReactiveX#6399

spec-dtslint/operators/concatMapTo-spec.ts

@@ -63,3 +63,7 @@ it('should enforce the return type', () => {
 it('should produce `Observable<never>` when mapping to an `ObservableInput<never>`', () => {
   const o = of(1, 2, 3).pipe(concatMapTo(Promise.reject())); // $ExpectType Observable<never>
 });
+
+it('should be deprecated', () => {
+  const o = of(1, 2, 3).pipe(concatMapTo(of(true))); // $ExpectDeprecation
+});

spec-dtslint/operators/mapTo-spec.ts

@@ -12,3 +12,7 @@ it('should infer correctly when returning a different type', () => {
 it('should enforce types', () => {
   const o = of(1, 2, 3).pipe(mapTo()); // $ExpectError
 });
+
+it('should be deprecated', () => {
+  const o = of(1, 2, 3).pipe(mapTo(true)); // $ExpectDeprecation
+});

spec-dtslint/operators/mergeMapTo-spec.ts

@@ -75,3 +75,7 @@ it('should enforce types of the concurrent parameter with a resultSelector', ()
 it('should produce `Observable<never>` when mapping to an `ObservableInput<never>`', () => {
   const o = of(1, 2, 3).pipe(mergeMapTo(Promise.reject())); // $ExpectType Observable<never>
 });
+
+it('should be deprecated', () => {
+  const o = of(1, 2, 3).pipe(mergeMapTo(of(true))); // $ExpectDeprecation
+});

spec-dtslint/operators/switchMapTo-spec.ts

@@ -58,3 +58,7 @@ it('should enforce the return type', () => {
 it('should produce `Observable<never>` when mapping to an `ObservableInput<never>`', () => {
   const o = of(1, 2, 3).pipe(switchMapTo(Promise.reject())); // $ExpectType Observable<never>
 });
+
+it('should be deprecated', () => {
+  const o = of(1, 2, 3).pipe(switchMapTo(of(true))); // $ExpectDeprecation
+});

src/internal/operators/concatMapTo.ts

@@ -2,7 +2,7 @@ import { concatMap } from './concatMap';
 import { ObservableInput, OperatorFunction, ObservedValueOf } from '../types';
 import { isFunction } from '../util/isFunction';
 
-/* tslint:disable:max-line-length */
+/** @deprecated Will be removed in v9. Use {@link concatMap} instead: `concatMap(() => result)` */
 export function concatMapTo<O extends ObservableInput<unknown>>(observable: O): OperatorFunction<unknown, ObservedValueOf<O>>;
 /** @deprecated The `resultSelector` parameter will be removed in v8. Use an inner `map` instead. Details: https://rxjs.dev/deprecations/resultSelector */
 export function concatMapTo<O extends ObservableInput<unknown>>(
@@ -14,7 +14,6 @@ export function concatMapTo<T, R, O extends ObservableInput<unknown>>(
   observable: O,
   resultSelector: (outerValue: T, innerValue: ObservedValueOf<O>, outerIndex: number, innerIndex: number) => R
 ): OperatorFunction<T, R>;
-/* tslint:enable:max-line-length */
 
 /**
  * Projects each source value to the same Observable which is merged multiple
@@ -70,6 +69,7 @@ export function concatMapTo<T, R, O extends ObservableInput<unknown>>(
  * @return A function that returns an Observable of values merged together by
  * joining the passed Observable with itself, one after the other, for each
  * value emitted from the source.
+ * @deprecated Will be removed in v9. Use {@link concatMap} instead: `concatMap(() => result)`
  */
 export function concatMapTo<T, R, O extends ObservableInput<unknown>>(
   innerObservable: O,

src/internal/operators/mapTo.ts

@@ -36,6 +36,7 @@ export function mapTo<T, R>(value: R): OperatorFunction<T, R>;
  * @param value The value to map each source value to.
  * @return A function that returns an Observable that emits the given `value`
  * every time the source Observable emits.
+ * @deprecated To be removed in v9. Use {@link map} instead: `map(() => value)`.
  */
 export function mapTo<R>(value: R): OperatorFunction<unknown, R> {
   return map(() => value);

src/internal/operators/switchMapTo.ts

@@ -2,7 +2,7 @@ import { switchMap } from './switchMap';
 import { ObservableInput, OperatorFunction, ObservedValueOf } from '../types';
 import { isFunction } from '../util/isFunction';
 
-/* tslint:disable:max-line-length */
+/** @deprecated Will be removed in v9. Use {@link mergeMap} instead: `mergeMap(() => result)` */
 export function switchMapTo<O extends ObservableInput<unknown>>(observable: O): OperatorFunction<unknown, ObservedValueOf<O>>;
 /** @deprecated The `resultSelector` parameter will be removed in v8. Use an inner `map` instead. Details: https://rxjs.dev/deprecations/resultSelector */
 export function switchMapTo<O extends ObservableInput<unknown>>(
@@ -14,7 +14,6 @@ export function switchMapTo<T, R, O extends ObservableInput<unknown>>(
   observable: O,
   resultSelector: (outerValue: T, innerValue: ObservedValueOf<O>, outerIndex: number, innerIndex: number) => R
 ): OperatorFunction<T, R>;
-/* tslint:enable:max-line-length */
 
 /**
  * Projects each source value to the same Observable which is flattened multiple
@@ -55,6 +54,7 @@ export function switchMapTo<T, R, O extends ObservableInput<unknown>>(
  * `resultSelector`) every time a value is emitted on the source Observable,
  * and taking only the values from the most recently projected inner
  * Observable.
+ * @deprecated Will be removed in v9. Use {@link mergeMap} instead: `mergeMap(() => result)`
  */
 export function switchMapTo<T, R, O extends ObservableInput<unknown>>(
   innerObservable: O,