opam update
opam switch create 5.0 --formula '"ocaml-base-compiler" {>= "5.0" < "5.1"}'
Official announcement: https://discuss.ocaml.org/t/ocaml-5-0-0-is-out/10974
To add some context: This is the first release with support for multicore and effect handlers(!). Multicore has been in progress for 8 years!