It was largely written in about a week, a few years ago. It's quite stale now unfortunately. It was updated to the current RFCs for mail, but it's still on a very old Calendars version.
The downside of the JMAP model for a proxy to IMAP is that you do need to have a bit of state in your proxy server. jmap-perl uses a sqlite3 database per user for storing that state.
Structurally, it's a non-blocking server in front of a forking server - the non-blocking handles connections and event streaming, and opens multiple backend forking servers per user, one for reads, one for writes, and one for background syncing/import.
https://jmap.io/software.html has a list of some known implementations.