Bitch ass Level 13.
It's been around for a long time, only for it to get deleted. The fact that there's no anomalous properties is no excuse, as a bunch of other levels are around which also aren't really anomalous, and you keep them.
I think Level ! should be removed, too. It's just where you get chased by entities, and you get sent there at random. It's just a gigantically inconvenient level with no reason to exist other than to add artificial difficulty to Backrooms survival.