Also infer RTBF semantics from "user ID no longer valid" (0x20) direct revocation
Dropping all other packets in the presence of a hard revocation direct signature is efficient, but there is another use case where people may want to invoke their right to be forgotten (by deleting personal data in User ID and User Attribute packets) but without the other side-effects of hard revocation (in particular, that historical signatures are retrospectively invalidated).
A UserID revocation sig cannot be validated in the absence of the original UserID packet, so it normally cannot be used for RTBF. However, we could make an 0x20 revocation direct sig that only depends on the primary key, similar to a hard revocation sig. In such a scenario we can infer that the UserID invalidation applies to all UserIDs, and delete them safely.
Open question: Do we want to still distribute subkeys in such a scenario? It may be desirable for people to be able to look up the primary key by subkey fingerprint (although this may not be strictly necessary, I haven't fully though this through...)