It is indeed true that the effect system will evolve in the future, and there is no syntactic support for effects in the 5.0 release. That said, the standard library in 5.0 does contain functions that will expose the same set of features, and can still be a very useful way to build things using effects. I'm hoping experiments in the wild will prove to be useful feedback as the effect system evolves in the future OCaml releases.
EDIT: https://discuss.ocaml.org/t/multicore-ocaml-september-2021-e... is a forum post that goes into more detail about the state of effects in OCaml 5.0