ReasonML is now Rescript, and is still using the 4.06 compiler. I think the idea is to move ahead largely independently of Ocaml, and that a move to 5.0 now is probably seriously ambitious given the runtime overhaul.
But not everyone agrees with the split and work is being done on Melange to replace Bucklescript : https://github.com/melange-re/melange
Ultimately JsOfOcaml can directly transpile Ocaml to JS.
Apparently it's not. The whole thing is a mess :)
https://ersin-akinci.medium.com/confused-about-rescript-resc...