It’s not as if Fastmail haven’t run JMAP in production at a pretty big level for quite some time by now.
To some degree, that isn't true anymore as of last week but of course, you meant when you used it, not generally: https://www.fastmail.com/blog/offline-in-beta/
Useless to me. Seriously.
I write a lot of email on planes and do a lot of email management.
All that works on the default Apple stuff as shipped other than account management. Using IMAP and iCal etc.