It is not, in fact, a standard. It's a proprietary complicating thing that megacorps do and everyone else assumes is standard.
https://datatracker.ietf.org/doc/html/rfc6749 "The OAuth 2.0 Authorization Framework"
>This specification is designed for use with HTTP ([RFC2616]). The use of OAuth over any protocol other than HTTP is out of scope.
So now you have HTTP protocol being used for IMAP, or worse and more common, not-OAuth over IMAP and you call that standard? These are Microsoft, Google, etc announcements of proprietary things. Not standards. And every single megacorp requires a different custom solution to interact with.