That proposal aims to allow type annotations as part of JS, but to do that the parser needs to know when a type annotation starts (easy) and ends (hard), the only realistic ways that can be done at the moment is explicitly supporting all of the syntax TS introduces (which is bad for other type checkers) or restricting to a limited subset which would mean a lot of what TS supports would not be allowed, negating the usefulness of the proposal.

What I'm saying is that if instead we said that all type annotations must follow existing JS grammar rules, with perhaps a couple of small additions, then we'd be able to support all sorts of complex type annotations in JS directly, using existing syntax. This would mean that TS's grammar would end up changing a bit, but that's a small price to pay for ongoing interoperability.

For example, instead of

    type UserProperties = keyof typeof User;

which is tied to TS's arbitrary syntax, and not supported in Flow, use

    type UserProperties = keyOf(typeOf(User));