Do you have a link to the proposal and the discussion? I am quite interested to see the implementation details. Thanks!
I'm the author. https://atgreen.github.io/repl-yell/posts/sbcl-fibers/
Here's an SBCL coroutines talk at the European Lisp Symposium from 2024: https://www.youtube.com/watch?v=S2nVKfYJykw
It's on the devel mailing list: https://sourceforge.net/p/sbcl/mailman/sbcl-devel/thread/CAF...