Essentially what Apple is doing is creating a new gate as they've done before with forcing developers to use their payment processing system for digital goods that costs 30% of the total fee.
Facebook Login already supports users blocking sending sites their email, as I disable that all the time. They even have published guidelines with what a site should do when they think they need that piece of information. So I'm not sold on the privacy story here. Go on, give it a try the next time you are signing in for the first time. You can even go back and remove your email permission from the security page in FB.
Apple did do one thing though, they created a bigger market for junk email services and made it easy to send those to services you expect to get notifications from. Still, this is something Apple could have built into iOS just like they did with password managers. It's not a feature that needed to be forced on anyone for any reason. As a user of the device, I can just pick, get me a junk email and presto, it's done.
I get it though, I can imagine the scenario last year right after news broke with Cambridge Analytica. This was enabled from a shady 3rd party developer using FB single sign-on and execs at Apple were pissed. They probably hastily demanded someone at Apple create something to circumvent FB's position. However, this doesn't just affect FB, it affects Twitter, Google, GitHub, Microsoft as well.
Yes Apple has good-will in this community, and its to their competitive advantage that they continue to build out more privacy focused features. However, I still do not appreciate that Apple gets the final say. This should have been a bigger conversation across the entire industry. Why didn't Apple come out and say, we are investing in the Oauth organization and then published papers on best practices? It would have been a much bigger win to get Microsoft or Google on board from the start and launched together. Hundreds of researchers all over the world would have jumped on it. Instead now we're left with another moral authority move, shoved in developers faces at the whims of Apple Execs.
Developers are free not to implement social login. If they do implement social login then this provides a unique user benefit (privacy) while also increasing user choice (users are still free to login how they want).