“OP_RETURN was always just bound by the maximum transaction size. We are not opening up anything now at the consensus level.”

- I assume you are referring to the fact that nodes can change their data carrier size to allow for 100kb. By default all core nodes prior v30 only allow for 80 + 3 bytes of op_return. The defaults are very important because most people run the defaults. This means that it is very hard to have your larger than 80 byte op_return file relayed across the p2p network, as most nodes do not relay this transaction. This also disincentives miners not to include this transaction as there is a higher risk of their block going “stale” if they do.

Regarding “the blame would be on the protocol either way” I’d say there’s a difference. If a bad actor must bypass the default P2P path by submitting directly to a cooperating miner or mining the block themselves, responsibility is clearly on the actor and the cooperating miner. If Core’s default changes so that most nodes relay large OP_RETURNs, that plausible-deniability vanishes because the network defaults now support large data relay, and the perception shifts toward the protocol. The anonymity of the miner/bad actor does not really matter in this case because whether the miner is known or not, it’s the perception of how that illicit data was included on chain and whether it was supported by the p2p network, or the network had to be bypassed because it did not support it.