People have gotten prohibited registrations for their AR-15s, but AFAIK it’s only the people who bought theirs before the ban and had not completed the transfer yet.
A good way to test the registration is for someone to convert their Modern Sporter to an SBR & tell the CFO to change it to a restricted rifle. Or at least call them and ask them if you still need to change the registration if you change it to an SBR. They will be forced to give you an answer.