Uh, email addresses aren't already public - there isn't some magic email directory in the cloud (I mean aside from those aggressively hoarded by trackers, advertisers, and marketers because you had no choice but to use your real address once).
But this is also specific to "social" logins - eg ones where you aren't intentionally providing your email address.
They're a public endpoint designed to intake communications from anybody. Identity is an outcome from people using the same email, and that's a valid reason for using a proxy, but the address itself has always been public.
But this is also specific to "social" logins - eg ones where you aren't intentionally providing your email address.