What's your definition of "allowed" then? Because a miner definitely can mine only empty blocks and a miner definitely can do overt ASICBoost (using as little as 1 bit of the version field) right now. I thought you meant allowed in the sense that if a block is allowed, it is a valid block on the network. It sounds like you mean something else, perhaps, "a block is allowed if it doesn't cause harm to the network." I'm not sure how you quantify that as that seems pretty subjective.