This is a big deal. One of the reasons services are so keen to require e-mail addresses from users with social logins is so that, if they drop that login service for whatever reason, users can recover access to their accounts using their email address. (A few major sites have done this recently, if I remember rightly.) This effectively eliminates that safeguard. They're not just forced to implement Apple Sign-In, but to make themselves dependent on it forevermore in a way that cautious companies have avoided.
