I thought the same thing about PGP/GPG keyservers. They are supposed to be immutable so if someone uploaded a key containing a jpeg attachment depicting illegal content, the only course of action would be to wipe every server in the network.
[...] we've had keyservers in Europe shut down because of privacy
demands because an append-only mesh-fill datastore can't remove keys and
people send out their email address and name paired into a key and then
get upset because it's out there; we're one
illicit-material-in-photo-uid incident away from global shutdown.
Indeed I was about to write as you have that key servers, or CT logs etc could arbitrarily decide not to include whatever keys they wanted -- although once included, it would be a pain to get rid of them because I think they're built on a Merkle tree structure which is routinely audited: I believe revoking a key doesn't actually delete a key from the database, but rather adds the statement "this key is now revoked", as it's an append-only ledger.
In theory the miners of a public blockchain could also do such auditing, to simply decide to ignore anything containing child pornography, just as they could arbitrarily decide to ignore any transactions. Of course, it's probably expensive to actually check all possible transaction in this way, and they most likely don't have a big enough incentive to. And of course for other sometimes illegal content (e.g. a picture of Mohammed or a particular nation's state secrets) some group of miners would censor while some wouldn't, and it would only take the miners who don't censor to win once for it to forever fork the blockchain.
Keyservers are convenient but they're not critical to PGP's functionality. There are other ways to distribute public keys. Actually in general public key servers are only mildly useful because you can't really trust anything you get from them, you have to use your web of trust of some other way to validate that the key is valid.
On the other hand if you wipe (even partially) Bitcoin's blockchain your basically destroy the currency.
