Mostly Fastmail but there are also a few open-source projects that support it https://jmap.io/software.html

It is used by Fastmail (who was also developers of the standard)

I think it is not a great idea to say that it is not a good idea to not implement things that no one uses, because that is a certain way to not make more people use it.

There are some servers that support it such as Stalwart [0], though I don't know about any older servers and/or clients that support it well.

[0]: https://github.com/stalwartlabs/jmap-server

Until Dovecot and systems like it support JMAP, I don't see it gaining the critical mass it needs to succeed.

Well I can't see Dovecot devs deciding to implement it if they keep reading comments like this dissuading the effort...

What advantage would they gain by implementing it?

There's three considerations

(a) is IMAP good / sufficient / fit for purpose

(b) if not, is there a better alternative available

(c) if there is, is it worth the development effort to support it.

If we assume the answers to (a) & (b) are NO & JMAP, then the benefits are included in those answers & only question remaining is (c). The person I was replying to seemed to be only arguing that the answer to (c) is NO.

(a) No (b) JMAP (c) Long term, probably.

Cyrus is a system like dovecot?

This is an chicken-egg problem.

> what's the point of implementing something that is not used at all

If this question made any sense we would never make anything new at all. How do you think new things get started?