There’s nothing in the PEP – or any other packaging spec that I can think of – that requires you support older versions of a spec.
Sure, but you don’t have to use it.
How about this: are you up to start a separate topic to discuss how to hash a directory of files? If you can reach consensus fast enough then I will incorporate the approach into the PEP (my guess is the PEP won’t get any accepted any sooner than next month).