If Gmail blocks me from making an account, I could use Yahoo mail or whatever, for email. But Gmail is more than just email. It's an entire identity. I can't use an android phone out of the box without a Google account. I ~bought~ licensed some movies from YouTube and those would just go poof. All the sites I used Sign in With Google would essentially be lost to me since I can't use that Google account.
The logical answer might be to just not use Sign in with Google but some services don't even let you use username/PW, it's sign in with Google/apple/Github etc.
Well yes, but that's partially on you − I might understand your average person not being wary about these issues, but we're on Hacker News here, these issues have been discussed for decades.
The sites also shouldn't be 'lost', don't they have a way to register another kind of login from the same one ? And you can always make a new, platform-free account − better do that sooner than later !
Please name and shame the websites (like Advent of Code and Rebble) that only allow a login through platforms. And services that only allow Google Authenticator.