Unused variables are an indication that the author hasn't completed their thought, so to speak, in the best case. In the worst case, it's a mistake and indicates the code is likely implemented in a way that it does something other than what it intended. I think making it a compiler error is the right way to do it. Other languages should adopt it.
The thing about software in development is that it isn't complete, practically by definition. I don't litter my code with unused imports and variables, I just have some stuff hanging out below while fixing it above. This is what linters are for, and unused variables and imports weren't the thing making software unmaintainable. Could even have a compiler flag that errors on unused for prod builds. There are a bunch of ways to skin a cat.
