mic That was just one of many refactorings, as is to be expected during the beta process.
What exactly are/were you using it for?
Authentication is now handled by two middlewares: AuthenticateWithSession and AuthenticateWithHeader. If any of these pass, they store the current user as "actor" in the request object, which can then be accessed in the controller using $request->getAttribute('actor'), for example.
Note that this will probably be changed again slightly in one of te coming betas.