That's a stage 1 proposal that has barely gained any traction since its release. In fact it hasn't been updated for quite a while (with "real" content changes). I wouldn't make decisions for my current code based on something that probably will never happen in the future.
https://github.com/tc39/proposal-type-annotations/commits/ma...
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TC-39 is kind of lame at the moment. I can’t imagine they will stay this lame forever. There are some reasonable voices inside TC-39, so even thought currently the lame voices at the committee are more powerful, that could change at any moment.