The OCaml version available from the Cygwin package manager is still at 4.14.2. I would kindly ask for an update. Please let me know if this is the wrong forum to ask.
In the meantime, might I suggest installing opam from the opam website? It runs in a cygwin environment and will install an up-to-date OCaml version for you (even though the script bootstraps from Powershell)
Yes, I know and it’s not a problem for me as a developer, but we have an environment where it is convenient if non-developers can compile OCaml source code, preferably without having to setup opam. I also think it could be good to signal with an update that OCaml 5 is restored on Cygwin.
Historically, packaging OCaml for the various distributions was done by various members of these communities.
For Cygwin, the list of the previous maintainers can be seen in the package’s git log. You can also send an email on the cygwin mailing-list to ask if anyone would be able to update it, or even send a patch to help the initial impulse.