Java 21, and I assume like every scripting language (Ruby, Python, etc). Though I guess with scripting you can't use more than one OS thread (not totally sure). Rust started off with it, and C# tried it too, but there are huge downsides to the model, so it's not like it's perfection incarnate and every other language just can't pull it off.
Does 21 actually automatically schedule fibers? to open cores?
Yup. It's all 100% automatic. Just use a different name for the thread pool and you're done, I believe.