I'm inclined to remove the export of *IMMUTABLE-SYSTEMS*. It hasn't been used in a released version of ASDF AFAIK, so it seems benign to remove it.
isn't that also the case for REGISTER-IMMUTABLE-SYSTEM?
if that export sticks in the release then it'll be a headache down the road (assuming that it is indeed an unfortunate name and not just my lone opinion).