Unison [1] has algebraic effects as a first-class feature. They call them "abilities" there. You can make async about as transparent/opaque as you want it. Docs on abilities in [2].
I don't know that anybody is running a company on it yet, no. They've got a couple big milestones coming down the pike, which might make it more attractive to people.
[1]: https://www.unison-lang.org/
[2]: https://www.unison-lang.org/learn/language-reference/abiliti...