Well, certainly string literal types have been around in GHC Haskell for quite a while (7.10.x). They've also been a thing in Scala 2.12.x although the 'implementation' was some sort of weird type checker 'hack'. I believe Scala 3 supports them natively.

I can't speak to the ergonomics of actually implementing anything which uses them internally since I've never really had much use for them outside using the surface-level API of a couple of libraries.