op_return is not part of a coinjoin tx but rather a part of tx0.

And transaction zero is what requires in zerolink protocol.

And zerolink protocol breaks all these parameters: deterministic links, unmixed change, mixing with the same participants, mixing with yourself.

Therefore ban is very targeted attack it is even worse because without tx0 one cannot enter whirlpool (coinjoin) and therefore banning a conjoin after all.