Yeah, Flow had the ambition to be sound but has never accomplished it.

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.)