"illegal pornography, can render the mere possession of a blockchain illegal."
I see this as a very strong legal attack vector on full nodes and cryptocurrencies, probably a way around it, is to only allow meta information on a cryptographic form, even then the owner can publish the view key publicly.
A drastic solution is to just prune or don't even allow metadata.
You don't even need the ability to record metadata on-chain to encode arbitrary data. An agreed-upon method of encoding it into ordinary transactions is enough. Even if BTC-style transactions were just inputs/outputs (they're not), you could still encode information down into the satoshi-place of the inputs or outputs themselves. It's even worse for something like Ethereum: essentially the whole point of that blockchain is to encode abritrary (executable) metadata in the form of the contracts themselves.
You could do that with an ordinary bank account though, and call the cops on your bank. In fact, you could do it with any service provider who logs your activity. Simply invent an encoding scheme and encode something illegal in your actions.
I see this as a very strong legal attack vector on full nodes and cryptocurrencies, probably a way around it, is to only allow meta information on a cryptographic form, even then the owner can publish the view key publicly.
A drastic solution is to just prune or don't even allow metadata.