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I was a bit confused and initially misunderstood this to be an announcement that Multicore OCaml was available now.

However, it seems to be an announcement that a future release - OCaml 5.0 - will support Multicore, Effect handlers etc ?

Yes, it's announcing that the next but one version, 5.0, will support multicore and effect handlers.

For what it's worth you can actually start using Multicore OCaml today, there are installation instructions on the wiki: https://github.com/ocaml-multicore/ocaml-multicore