That might be why we found absolutely no bogus ADK packets when we scanned all the public keys on the public key servers after the bug was reported, even after nearly three years of deployment of this bug. If this were a back door, it would be a particularly stupid design for a back door. The fixed version of PGP deletes bogus ADKs wherever it finds them. It does not simply avoid using the bogus ADKs, it actually deletes them from the key. They are gone.
Some people are impossible to please.
We also made a change to the freeware version of PGP 7.0 to make it impossible for the user to turn off warnings about the presence of an ADK on a key before using it. In both the commercial and freeware PGP 7.0, we also made the warnings to the user far more explicit when encrypting to a key with the ADK feature. We fixed these problems at the same time, in the same way. In fact, while we were examining the source code to fix the designated revoker feature, we discovered another bug relating to designated revokers and fixed that too. That bug allowed invalid designated revoker signatures to be accepted under some circumstances.