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.
So it's Reason, not ReasonML which the umbrella project's name, and Rescript is a imcompatible syntax split from the Bucklescript team (that previously transpiled Reason to JS). Bucklescript's new name is... Rescript.
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.