That's irrelevant. You don't kill a service like that without a public deprecation plan and a lot of communication. Both users and services that relied on it need time to migrate.
This is what normal enterprise facing business do, is really unacceptable that Twitter did that without notice, we normally go through a legal and communication process before we turn off the lights
I don't know about that. I've seen enough websites that have specifically removed social logins, so it appears that there is an awareness that the feature was "strings attached" kind of thing. And no, these are all tech oriented sites I've seen this on either.
At Twitter's scale "very few" means "a couple million"