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.