docs: list IE 9 and 10 as deprecated (#36887)

Add documentation in the deprecations markdown file about the deprecation of IE 9 and 10.
Additionally, add note in browser support document about deprecation.

PR Close #36887

aio/content/guide/browser-support.md

@@ -53,7 +53,8 @@ Angular supports most recent browsers. This includes the following specific vers
       IE
     </td>
     <td>
-      11, 10, 9 ("compatibility view" mode not supported)
+      <div> 11, 10*, 9* ("compatibility view" mode not supported) </div>
+      <div>*deprecated in v10, see the <a href="/guide/deprecations#ie-9-10">deprecations guide</a>.</div>
     </td>
   </tr>
  <tr>

aio/content/guide/deprecations.md

@@ -60,6 +60,7 @@ v9 - v12
 | `@angular/core/testing`       | [`TestBed.get`](#testing)                                                     | <!--v9--> v12 |
 | `@angular/router`             | [`ActivatedRoute` params and `queryParams` properties](#activatedroute-props) | unspecified |
 | template syntax               | [`/deep/`, `>>>`, and `::ng-deep`](#deep-component-style-selector)            | <!--v7--> unspecified |
+| browser support               | [`IE 9 and 10`](#ie-9-10)                                                     | <!--v10--> v11 |
 
 
 
@@ -562,7 +563,17 @@ In practical terms, the `package.json` of all `@angular` packages will change in
 
 For more information about the npm package format, see the [Angular Package Format spec](https://goo.gl/jB3GVv).
 
+{@a ie-9-10}
+### IE 9 and 10 support
 
+Support for IE 9 and 10 has been deprecated and will be removed in a future version.
+Supporting outdated browsers like these increases bundle size, code complexity, and test load, and also requires time and effort that could be spent on improvements to the framework.
+For example, fixing issues can be more difficult, as a straightforward fix for modern browsers could break old ones that have quirks due to not receiving updates from vendors. 
+
+The final decision was made on three key points:
+* __Vendor support__: Microsoft dropped support of IE 9 and 10 on 1/12/16, meaning they no longer provide security updates or technical support.
+* __Usage statistics__: We looked at usage trends for IE 9 and 10 from various sources and all indicated that usage percentages were extremely small (fractions of 1%).
+* __Feedback from partners__: We also reached out to some of our Angular customers and none expressed concern about dropping IE 9 and 10 support.
 
 {@a removed}
 ## Removed APIs