We have a nice migration guide for the AccentColor change, but it's not linked in the API itself:
https://flutter.dev/docs/release/breaking-changes/theme-data-accent-properties#migration-guide

In this case, we include a brief warning ("'accentColor' is deprecated and shouldn't be used. Use colorScheme.secondary instead. This feature was deprecated after v2.3.0-0.1.pre..") but there's no help for a user to know how to use colorScheme, and it turns out to be pretty intricate.

I had to go off and Google to find the solution, and I think it could and should be a lot easier for us to walk users through this kind of breakage.