I haven’t been working on the JMAP stuff (I only started working there this year and I’ve been focusing on our new product Topicbox), so I’m not aware of all the details; but JMAP standardisation is going through the IETF, so you can see things that are happening with it at https://datatracker.ietf.org/wg/jmap/documents/ and https://github.com/jmapio/jmap/issues.