OCamlot is, as you saw, experimental software and anyone running it in production would basically be taking on all the risk and support burden themselves.
If you want a communication platform that should not ever go down, the only thing that can possibly deliver that is the BEAM platform i.e. Erlang/Elixir (used by WhatsApp backend): Pleroma / pleroma · GitLab
It’s questionable why you want something in OCaml specifically? I’m not aware of any such system implemented in OCaml, and, furthermore, besides an ideological win of using OCaml in the backend, it’s not clear why these existing platforms wouldn’t work.
Alternatively, if you want to self host, there are options like Zulip, Matrix, Mattermost etc which are battle-tested and used for several large communities. These are mostly written in Python, but using tools like pyml, you can easily write OCaml code that integrates with them.
Although, that being said, I wouldn’t recommend Activitypub as a protocol for @Luc_ML 's purposes. The key benefit of Activitypub is being able to interoperate with other servers — in this case, the features being requested (a main discussion thread, documents storing, side threads), are really orthogonal to the features provided by Activitypub, so it’s probably better to stick to a system dedicated to those features.