Gusto adopted it when I was there (I left at the end of 2022). Like other comments, it's a bit polarizing but I found it to be useful when working in large codebases because you one didn't have to reason about return types and could just read them from function signatures.
Getting it to play nicely with Rails and other gems that did metaprogramming magic was a constant pain in the butt though.
Getting it to play nicely with Rails and other gems that did metaprogramming magic was a constant pain in the butt though.