fix(material/tabs): deprecate backgroundColor API (#28262)

(cherry picked from commit 771f1df)

Author: andrewseguin

src/material/tabs/tab-group.ts

@@ -69,6 +69,9 @@ export interface MatTabGroupBaseHeader {
 /** Possible positions for the tab header. */
 export type MatTabHeaderPosition = 'above' | 'below';
 
+/** Boolean constant that determines whether the tab group supports the `backgroundColor` input */
+const ENABLE_BACKGROUND_INPUT = true;
+
 /**
  * Material design tab-group component. Supports basic tab pairs (label + content) and includes
  * animated ink-bar, keyboard navigation, and screen reader.
@@ -236,13 +239,21 @@ export class MatTabGroup
 
   private _preserveContent: boolean = false;
 
-  /** Background color of the tab group. */
+  /**
+   * Background color of the tab group.
+   * @deprecated The background color should be customized through Sass theming APIs.
+   * @breaking-change 20.0.0 Remove this input
+   */
   @Input()
   get backgroundColor(): ThemePalette {
     return this._backgroundColor;
   }
 
   set backgroundColor(value: ThemePalette) {
+    if (!ENABLE_BACKGROUND_INPUT) {
+      throw new Error(`mat-tab-group background color must be set through the Sass theming API`);
+    }
+
     const classList: DOMTokenList = this._elementRef.nativeElement.classList;
 
     classList.remove('mat-tabs-with-background', `mat-background-${this.backgroundColor}`);

tools/public_api_guard/material/tabs.md

@@ -266,6 +266,7 @@ export class MatTabGroup extends _MatTabGroupMixinBase implements AfterContentIn
     set animationDuration(value: NumberInput);
     // (undocumented)
     _animationMode?: string | undefined;
+    // @deprecated
     get backgroundColor(): ThemePalette;
     set backgroundColor(value: ThemePalette);
     get contentTabIndex(): number | null;