Flow's type system isn't sound either FWIW
If you read the Flow codebase and its Git history, you can see that it's not for lack of trying, either — every couple of years there's an ambitious new engineer with a new plan for how to make it happen. But it's a real tough migration problem — it only works if they can provide a credible, appealing migration path to the other engineers across Facebook/Meta's giant JS codebase. Starting from a language like JS with all the dynamic tricks people use there, that's a tough job.
(And naturally it'd be even harder if they were trying to get any wider community to migrate, outside their own employer.)
I wonder how widely used that option is. As I said in that other comment, it feels to me like the sort of thing that would produce errors all over the place, and would therefore be a real pain to migrate to. (It'd be just fine if the language semantics were that out-of-bounds array access throws, but that's not the semantics JS has.) I don't have a real empirical sense of that, though.