Or ... TypeScript developers could just have added test cases to their test suite and figure out automatically whether the new implementation fixed things or caused regressions.
I mean, really? Less than 600 bugs, and they couldn't be bothered? Even with a small team. this could have been done in a day.
And it's not like it would be extra work, because going through the bug tracker and trying to get a broader view of the issues raised should be a thing which happens continually anyway.