Since the appetite of the language steering group was mentioned here, we did indeed discuss this pitch and in particular conditionally rejecting code based on -enable-library-evolution or a similar flag.
Today, we have one place where -enable-library-evolution changes the semantics of the language in a very specific way, and it has caused an outsized amount of pain and confusion. Moving forward, we want to avoid introducing more such features.
As pitched, enabling library evolution mode (or similar) would make the language more permissive than without the mode, and we don't think that users should have to opt into such a mode in order to write reasonable code.
Therefore, since we do think there's at least one--hard, but implementable--alternative design which doesn't require creating a new language dialect, we would concur with feedback that the pitched design wouldn't be our preferred approach to the problem--although of course we do certainly appreciate all of the effort and motivation!
-- Xiaodi, on behalf of the language steering group