Point to Sinon 9 for legacy support

COMPATIBILITY.md

@@ -4,13 +4,14 @@ This file is the canonical resource for compatibility goals for the libraries th
 
 ## ES2017
 
-The source is as [ES2017][es2017] and requires no transpiler or polyfills in supporting runtimes.
+Sinon has historically been written as [ES5][es5], but starting from Sinon 10 we made the breaking change to allow more modern ECMAScript versions, starting with ECMAScript [2017][es2017].
+The source version is [ES2017][es2017] and requires no transpiler or polyfills in supporting runtimes.
 
-For legacy runtimes, a transpiler can be used by dependent projects.
+For legacy runtimes, a transpiler can be used by dependent projects., but use of newer APIs might break. If you need to support older browsers, have a look at Sinon 9.
 
 ## Runtimes
 
-Sinon.JS aims at supporting the following runtimes:
+Sinon.JS aims at supporting the following runtimes (from our shared [eslint-config][shared-config]).
 
 <!-- browserslist start -->
 
@@ -34,4 +35,6 @@ To see what that means in practice, you can use https://browserslist.dev
 <a href="https://saucelabs.com/u/sinonjs"><img src="https://saucelabs.com/browser-matrix/sinonjs.svg" alt="Sauce Test Status"></a>
 </p>
 
+[es5]: http://www.ecma-international.org/ecma-262/5.1/
 [es2017]: https://262.ecma-international.org/8.0/
+[shared-config]: https://github.com/sinonjs/eslint-config

docs/release-source/release.md

@@ -25,24 +25,14 @@ This page contains the entire Sinon.JS API documentation along with brief introd
 
 {% include docs/migration-guides.md %}
 
-## Compatibility
-
-The most up-to-date reference on which runtimes and browsers we support can be found by looking at our shared [eslint-config][shared-config].
-
-### ECMAScript versions
-
-Sinon has historically been written as [ES5][es5], but starting from Sinon 10 we made the breaking change to use more modern ECMAScript versions, starting with ECMAScript [2017][es2017]. `{{page.release_id}}` requires no transpiler or polyfills to run in the runtimes mentioned below.
-
-### Supported runtimes
+## Compatibility and supported runtimes
 
 As of Sinon 10 we stopped maintaining compatibility with legacy browsers. Instead, we focus on compatibility with evergreen browsers, [Node.js LTS versions](https://github.com/nodejs/Release) and recent Safari versions.
+The most up-to-date reference on which runtimes and browsers we support can be found by looking at our [compatibility docs][compat-doc].
 
-There should not be any issues with using Sinon `{{page.release_id}}` in newer versions of the same runtimes.
-
-If you need to support old runtimes you can try one of the older Sinon versions.
+If you need to support old runtimes you can try [Sinon 9][compat-doc-v9].
 
 {% include docs/contribute.md %}
 
-[es5]: http://www.ecma-international.org/ecma-262/5.1/
-[es2017]: http://www.ecma-international.org/ecma-262/8.0/
-[shared-config]: https://github.com/sinonjs/eslint-config
+[compat-doc]: https://github.com/sinonjs/sinon/COMPATIBILITY.md
+[compat-doc-v9]: https://github.com/sinonjs/sinon/blob/v9.2.4/COMPATIBILITY.md