But I don't see any problems at the moment.
No method currently explicitly reports void.
The user mentions AppController::beforeFilter(), but that's not explicit either.

If we want to proceed in this direction, it should be made explicit everywhere (all events of all classes that extend Controller) ???

This will happen, but for BC reasons we cannot change the core (in a patch) release. Generated code and this boilerplate code can be adjusted safely usually.

Yes, this is totally fine to do in the skeleton.