They are actively working on it (https://discuss.ocaml.org/t/multicore-ocaml-may-2020-update/...) (HN discussion here, https://news.ycombinator.com/item?id=23380370).

They're definitely making progress lately. For a few years there it seemed like it would never happen, but now I'm starting to have some hope.

What will you use multicore support for?