After migrating to UIScenes, registering plugins in the application(_:didFinishLaunchingWithOptions:) function is no longer recommended. It will still work by using a new mechanism called FlutterLaunchEngine.

We'd like to eventually stop supporting FlutterLaunchEngine, though.

As such, Flutter apps will need to migrate their AppDelegate.swift / AppDelegate.m.

Flutter should auto-migrate any AppDelegates that are exact matches to the old templates. If unable to auto-migrate and it's detected that the FlutterLaunchEngine is being used, Flutter should then show a runtime warning that points to docs on how to migrate manually.