I feel like this fork might make sense inside Bloomberg, but it’s killed my enthusiasm for this language family.
The messaging isn't clear on ReasonML's future at ALL though. Is it essentially deprecated until the full switch to ReScript happens? The Rescript FAQ mentions changes will be additive in supporting ReasonML, and they will track new features to OCaml, but will they track new features to Reason? Will anyone actively develop Reason anymore?
The success of BuckleScript meant that a _lot_ of people used what they thought was ReasonML but it was really ReasonML+BuckleScript. This lead to a ton of confusion the moment newcomers realised they needed a second website to look up APIs.
ReScript includes a snapshot version of ReasonML for backwards compatibility, but it will never be upgraded.
The future of ReasonML is likely slow and steady progress, potentially fading into obscurity as the target audience is now very small (OCaml developers who don't like OCaml syntax).
To write the same code for both front and back end js_of_ocaml has been the standard since long before Reason came along, and it works just fine with Reason native.
To have code that you can compile to both native and js, the easiest thing is to stick to the ocaml syntax. It is supported by all the versions of the compiler (rescript or ocaml). And to also use the ocaml syntax for the tools doing code generation, such as atdgen.