AuthManager has a bit of a chicken-and-egg problem: it cannot land before extensions (at least WMF-deployed ones) are updated to be compatible, and extensions cannot be updated before AuthManager has landed.
Fore directly auth-related functionality (providing and using $wgAuth) the compatibility wrappers take care of this, but the problem still remains for the changes in User - see T110433. (Also for the ResetSessionID hook which is completely removed, but at least on gerrit nothing uses that one.)