They have an effects branch but admitted it won't be ready in time for OCaml 5.0 (supposedly arriving around end of 2021 / early 2022) so they have a separate compiler variant with it where the progress can be tracked.

Last I read, they really want to nail down the entire multi-threaded environment to the last detail first (which mostly means make sure no parallel bugs are introduced + single-threaded code won't become slower; both pretty huge goals).