Great job on the PEP! My apologies for being late to the party here.
Overall I am extremely against this proposal because I think we should introduce a new field which will deprecate extras, hopefully called features, that allows us to ignore backward compatibility and focus on what is best for users. Has this been considered? If not I think this should be in the rejected ideas.
edit: also I’d be willing to write a PEP for that if no one else wants to