Cyrus will have support this year. It already has fairly complete calendar and contacts support, and mail support is being worked on as we speak.
https://cyrusimap.org/imap/developer/jmap.html
I've heard rumours from three different directions that Dovecot are planning support, but I haven't been able to substantiate that.
Then there's the proxy, which is seriously intended to be part of a migration strategy for existing servers that don't yet have support.
In reality, I don't think I'd expect much past exploration from most places until the protocol hits 1.0 - it's probably changing just a little bit too fast for anyone to commit strongly to just yet.