Not only that, but since it isn't required many libraries in the ecosystem won't be using it. Comparing my experiences with TypeScript and Flow vs. my experiences with Haskell, any optional type system is going to leave a lot of gaps if you don't have a pretty strict not-invented-here culture.
If you enter any random TS project today, odds are it isn't using that switch, and enabling the switch will lead you down a month long rabbit-hole of fixing cascading type errors.