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...
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.
OP_RETURN was originally intended to be a testing opcode and wasn't meant to be used on chain.