Upcoming OCaml multicore won't expose effects at all. I believe the intention is to finish work on typed effects[1] and only then add it to the language.
[1]: https://www.janestreet.com/tech-talks/effective-programming/
[1]: https://www.janestreet.com/tech-talks/effective-programming/
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).