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Re:Why we won’t be supporting Sign in with Apple by ourcat on Monday June 29, 2020 @07:52PM (#23684903)
> "I really wish there were a way to know whether I already used Facebook, Google, or Apple to log into a site or app before"

You can. On each of the places you mention (Google and Facebook, certainly), somewhere in a settings page/window, you'll find your list of 'authorised apps'.

These will be a list the login systems to the third-party sites you've used to log in with.

You should then see a way to 'revoke' their access to your data.

Re:Why we won’t be supporting Sign in with Apple by jdxcode on Monday June 29, 2020 @07:59PM (#23684961)
That doesn't solve the problem. You would have to go to every method to figure out which one it might be listed under. It needs to be in the browser/password manager to tell you which service was used.