Hi! I'm the co-author of the Client ID Metadata Documents internet draft.

It offers an alternative to Dynamic Client Registration (DCR), where instead of having to keep track of client registrations, you instead use an URI as your client_id and that URI points to a JSON document containing all the same DCR properties, which the authorization server will fetch if needed to register a client.

It's already in use by Bluesky / AT Proto, and probably has some use case here too. I'm also working soon on a Doorkeeper plugin to support it there too.

(Note: the URI as client_id format is changing a little bit with the Client ID Prefix I-D)