With the convergence between the multicore and standard runtime across OCaml 4.10.0 to 4.13.0 , the development of OCaml multicore has reached a point where further integration into OCaml’s main branch requires fully committing to a switch to OCaml multicore.
The OCaml team has decided that the time has come for such a commitment. The new major version, OCaml 5, will be a multicore version of OCaml. Moreover, OCaml 4.14 will be the last minor release of the 4.x series of OCaml.
The first version of OCaml multicore, code-named OCaml 5.0, will be a Minimum Viable Product focused on:
- Linux, MacOS, Windows mingw-w64
- Parallelism through Domains 
- Concurrency through Effect Handlers  (without syntactic support and exposed as functions from the standard library )
Our plan is to integrate the multicore branch into the main branch during the next 6 months. Hopefully, OCaml 5.0 will then be released between March and April 2022.
Note that OCaml 5.0 focuses on minimal (solid) support for the multicore runtime system, and will not provide stable user-facing concurrency and parallelism libraries. There has been a lot of experimentation [3,4,6] in the last few years, and some work remains to offer long-term, user-facing concurrent and parallel programming abstractions. OCaml 5.0 will be a great time to start adding concurrency and parallelism to your OCaml programs, but the libraries will still be in flux.
While OCaml 5 is stabilising, we plan to extend the support period for OCaml 4.14 by publishing minor bugfix releases whenever needed. In particular, OCaml 4.14 will be supported until all tier-1 architectures and operating systems are available in OCaml 5, and OCaml 5 sequential performance is close enough to that of OCaml 4.
To ensure that maintainers can concentrate on Multicore integration, and avoid any rebase work for the Multicore developers, the trunk branch will be feature-frozen starting from November 2021. All non-bugfix non-multicore contributions will be delayed to after the Multicore integration. We are calling this period the “sequential glaciation”.
We understand that this may be frustrating for our contributors, and apologize for the delay in getting your nice work reviewed and merged into the codebase. We hope that the sequential glaciation will be a good opportunity to help with the Multicore integration, review and testing, and/or focus on non-core-compiler efforts and the rest of the OCaml ecosystem.
With this early feature-freeze, we also plan to release OCaml 4.14.0 in advance, between January and February 2022, reducing the concurrency between the OCaml 5.0 and OCaml 4.14.0 releases.
 “Retrofitting Parallelism onto OCaml”, ICFP 2020, [2004.11663] Retrofitting Parallelism onto OCaml
 “Retrofitting Concurrency onto OCaml”, PLDI 2021, [2104.00250] Retrofitting Effect Handlers onto OCaml
 Domainslib – Parallel Programming over Multicore OCaml, GitHub - ocaml-multicore/domainslib: Parallel Programming over Domains
 eio – Effects-based Parallel IO for OCaml, GitHub - ocaml-multicore/eio: Effects-based direct-style IO for multicore OCaml
 Effect handlers in OCaml 5.0: https://discuss.ocaml.org/t/multicore-ocaml-september-2021-effect-handlers-will-be-in-ocaml-5-0
 Multicore monthly, Topics tagged multicore-monthly