Mention PR #5962 in CHANGELOG.md and comments.

CHANGELOG.md

@@ -32,6 +32,9 @@
 - **[BREAKING]** We are no longer exporting certain (intended to be) internal utilities. If you are depending on some of the lesser known exports from `apollo-cache`, `apollo-cache-inmemory`, or `apollo-utilities`, they may no longer be available from `@apollo/client`. <br/>
   [@hwillson](https://github.com/hwillson) in [#5437](https://github.com/apollographql/apollo-client/pull/5437) and [#5514](https://github.com/apollographql/apollo-client/pull/5514)
 
+- **[BREAKING?]** Remove `fixPolyfills.ts`, except when bundling for React Native. If you have trouble with `Map` or `Set` operations due to frozen key objects in React Native, either update React Native to version 0.59.0 (or 0.61.x, if possible) or investigate why `fixPolyfills.native.js` is not included in your bundle. <br/>
+  [@benjamn](https://github.com/benjamn) in [#5962](https://github.com/apollographql/apollo-client/pull/5962)
+
 - `InMemoryCache` now supports tracing garbage collection and eviction. Note that the signature of the `evict` method has been simplified in a potentially backwards-incompatible way. <br/>
   [@benjamn](https://github.com/benjamn) in [#5310](https://github.com/apollographql/apollo-client/pull/5310)
 

src/cache/inmemory/fixPolyfills.ts

@@ -4,4 +4,5 @@
 // When bundling for React Native, we substitute fixPolyfills.native.js
 // for fixPolyfills.js (see the "react-native" section of package.json),
 // to work around problems with Map and Set polyfills in older versions of
-// React Native (which should have been fixed in [email protected]).
+// React Native (which should have been fixed in [email protected]):
+// https://github.com/apollographql/apollo-client/pull/5962