Earlier quoted context omitted.
"all their arguments apply to all third-party sign-ons" No they don't. Other sign-on options don't obfuscate the email address. They are likely removing FB login as otherwise their next app update will be rejected by Apple for supporting third party login but not Apple login.
As a user I can't see why any random app needs to know my true email address.
I'm not saying this is necessarily a good thing, but this is how things work and I don't have a better suggestion.