Agree with the comment largely; though: Apple does have a policy where apps need to use SIWA if other oauth providers are present (Google) [0]. So sites do "opt in" to this, but are forced by apple to use SIWA if they're using any other provider.
reply[0]: https://developer.apple.com/app-store/review/guidelines/#sig...
Not arbitrarily many aliases - there's a limit of (as of two or three weeks ago) 768.
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