The runtime aspects of the multicore runtime has been pretty stable. Most of the effort currently is going into the algebraic effects extension that is used to map direct-style concurrency into multiple (parallel) cores: https://github.com/ocamllabs/ocaml-effects

That doesn't mean there isn't work taking place. Here's a recent video where KC talks about it.

https://m.youtube.com/watch?list=PLnqUlCo055hU46uoONmhYGUbYA...

Edit: and there's also more than one repo out there.