Abandoned is perhaps too strong a term :-). Effect handlers in the language are supported by fibers, lightweight stacklets managed by the runtime. The details of the implementation can be found in the upcoming research paper in PLDI'21 conference [1]. The effect handlers in Multicore OCaml today do not provide effect safety. Programs are not statically guaranteed to handle all the effects they may perform. This is only as bad as exceptions in OCaml and every other mainstream language with exceptions.

We are working on developing an effect system, which will ensure effect safety i.e, the compiler ensures that all the effects performed are caught. You also get a nice inferred type that says what effects a particular function may perform; if it performs none, then it is a pure function! This implementation would still use the current fiber support in the runtime. Leo White, one of the developers of Multicore OCaml had given a talk on this new effect system a few years ago [2]. That's the best place today to learn about the new effect handlers.

The plan is to first add the fiber runtime support to OCaml without the syntax extensions for effect handlers, and then introduce syntax along with the effect system.

[1] https://arxiv.org/abs/2104.00250

[2] https://www.janestreet.com/tech-talks/effective-programming/