I imagine all Haskell frameworks do; the ones I've tried surely do. Haskellers are accustomed to mixing string types, since the default String type is inefficient (a linked list of Char) and most import a library to provide immutable ~Pascal strings. And since this is such a common occurrence, the syntax has support form multiple string literal types. An application developer can literally create their own. It's also trivial to create a new type in Haskell that has minimal runtime cost, so it's pretty harmless.

I think if you don't have an easy way of creating string literals in the type you want, the developers will at some point reach for the deprecated api, and at that point you're just requiring good hygiene. Which is exactly what you're trying to stop. Language support is critical in being able to get away with this.