Update text to emphasise that the target is moving

Author: fatso83

COMPATIBILITY.md

@@ -2,10 +2,9 @@
 
 This file is the canonical resource for compatibility goals for the libraries that make up Sinon.JS.
 
-## ES2017
+## Moving target: ES2017+
 
-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.
+Sinon has historically been written as [ES5][es5], but starting from Sinon 10 (March 2021) we made the breaking change to allow more modern ECMAScript versions, starting with ECMAScript [2017][es2017]. This is not a fixed level: it will change in time without us updating it here. The source version requires no transpiler or polyfills in supporting runtimes.
 
 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.