Note that this is already happening. The only thing my proposal would change is that it would also affect servers that used to be authoritative for subdomains o

Note that this is already happening. The only thing my proposal would change is that it would also affect servers that used to be authoritative for subdomains of such abandoned domains. I would expect there to be very few of them: very few domains have delegations of subdomains to a different DNS server and they are larger and thus less likely to be abandoned.