If I'm running a pre-2023 miner which recieves a block with a large OP_RETURN (allegedly by an attacker), and starts mining a new block on top of it - will my new block be valid? No, because I did not update my miner's code. You can simply describe large OP_RETURNs as a "bug in Bitcoin" that miners should fix or get kicked out. There's no need for confusing language.

That's just not true. Prior to the block height of activation all transactions currently within consensus are valid. It is only once RDTS is activated that blocks will be thrown out by other nodes. OP_RETURN was originally intended to be a testing opcode and wasn't meant to be used on chain.

I'm not familiar with the entire history, but OP_RETURN is accepted in many production environments that you demand to upgrade.

I am not demanding anything. I am telling you technical details about how Bitcoin works.