Update compatibility docs

COMPATIBILITY.md

@@ -4,13 +4,16 @@ 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 -->
 
 ```
 > 0.5%
@@ -22,6 +25,8 @@ not op_mini all
 maintained node versions
 ```
 
+<!-- browserslist end -->
+
 The compatibility is enforced using [`eslint-plugin-compat`](https://www.npmjs.com/package/eslint-plugin-compat).
 
 To see what that means in practice, you can use https://browserslist.dev
@@ -30,4 +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

@@ -25,28 +25,14 @@ This page contains the entire Sinon.JS API documentation along with brief introd
 
 {% include docs/migration-guides.md %}
 
-### Compatibility
+## Compatibility and supported runtimes
 
-### ES5.1
+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].
 
-Sinon `{{page.release_id}}` is written as [ES5.1][es5] and requires no transpiler or polyfills to run in the runtimes listed below.
-
-### Supported runtimes
-
-`{{page.release_id}}` has been verified in these runtimes:
-
-- Firefox 45
-- Chrome 48
-- Internet Explorer 11
-- Edge 14
-- Safari 9
-- [Node.js LTS versions](https://github.com/nodejs/Release)
-
-There should not be any issues with using Sinon `{{page.release_id}}` in newer versions of the same runtimes.
-
-If you need to support very old runtimes that have incomplete support for [ES5.1][es5] you might get away with using loading [`es5-shim`][es5-shim] in your test environment.
+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/
-[es5-shim]: https://github.com/es-shims/es5-shim
+[compat-doc]: https://github.com/sinonjs/sinon/COMPATIBILITY.md
+[compat-doc-v9]: https://github.com/sinonjs/sinon/blob/v9.2.4/COMPATIBILITY.md