Overall, I’m in favour of this because it would enable the user authorization workflow in the ActivityPub plugin and beyond without much friction for admins.
Some questions though:
I’m not a huge fan of the migration here, I would prefer to leave existing user api keys as they are because the id and name of individual user keys are independent from the id and name of the registered clients. Maybe instead we can add a column to user_api_key for user_api_key_client_id. The keys with associated clients can then use the redirect configured by the client (and existing keys, of course, keep the current behaviour).
Further, should admins be able to review somewhere the list of registered clients?