RFC 8414 support is coming in Mastodon 4.3!
It didn't make the trunks and tidbits today, but this allows you to hit a well-known endpoint and retrieve back a list of OAuth Scopes, and various other URLs that are useful when writing a third-party application for Mastodon.
GET /.well-known/oauth-authorization-server on Mastodon 4.3 will give you everything* you need to know to interact as an OAuth 2 Client.
https://github.com/mastodon/mastodon/pull/29191#event-12714997119