If you mean GHC, the language version is whatever the pile of configuration flags at each source file ends up meaning, some of them even contradict themselves.
Great for language research, which is Haskell main purpose in life, hardly a good idea for getting industry love.
I am talking about Haskell, specifically the removal of 'n+k patterns' and 'monad comprehensions'.
About GHC: I think their approach with pragmas is great and something for other languages to emulate. It's also great in production, with the caveat that you might want to restrict that mechanism to surface level changes only, and nothing that changes the intermediate format.
