I’ve implemented the proposal in pyright. It was thankfully easier to implement and had much less of an impact on type checking results than I expected. Here is the mypy_primer output, which shows that pyright users were using # type: ignore to work around its old behavior.
I agree with Jelle that the typing council should consider this proposal for formal inclusion in the typing spec.
Mypy would need to make a (hopefully minor) change to conform with the proposal.