It fixes a critical bug on the authorization code grant. Hoping you did not encounter it, this version looks to be ok. Leave a note if you have any issues.
This version supports Self-Sovereign Identity abilities with the issuance of verifiable credentials and improving security with Pushed Authorization requests and Demonstration of Proof-of-Possession.
For now, there isn’t. You can setup a one-one relationship with an other table to have additional data to the clients.
That said, depending on your use case, there is the possibility to add an additional_data field to the clients if it makes sense. You can create a GitHub issue if you think it is relevant to go on the discussion there.