This isn't a lore dump so much as my thoughts on the matter.
Seeing as liminality is a central component of the Backrooms as a concept, it stands to follow that its associated Simulation Theory is strong, in-context.
Let's assume for a second that the latest data that disproves this theory never popped up. All we have is based on the stability of most gathered information, on the notion that physics don't appear to be arbitrary, and that there would even be a need for a space akin to the Backrooms as part of the structure of our world. With that assumption in mind, we can imagine that every rock texture, every ice formation, every patch of skin, work of art or street corner - literally everything that's existed or could exist - is an asset being exploited within the context of the Universe's wider rendering pipeline. If you start at that point, then it's obvious that time renders some assets obsolete and that by our reckoning, the best way to effectively delete them is through demolition. Yesteryear's neon-infused megamalls get torn down, open-air commercial strips get put up and surrounded by McMansions - but the megamalls don't stop existing as a concept. The megamalls become akin to the concept of a tulpa that gained some ground during the Slender Man years - the idea of thought-forms or effectively reified and signified objects.
So where is the concept of the Megamall stored, unformed and without concrete manifestations - so consequently doomed to exist in a state of un-being and of constant redefinition when confronted with other obsolete realities? That particular somewhere would be the Backrooms.
Put up a concept, erect its best-available representation, and my thinking is you've planted a seed in the Backrooms. I think the idea of just tossing whatever at the walls and calling it a "level" is cheating the concept of its true depth. Maybe if you go deep enough inside its maze, the yellow wallpaper and beige carpet stop and bleed into peeling wainscoted walls, which themselves bleed into joined stones and an endless enfilade of corridors ripped out of the core Concept of the Medieval Keep. Go deep enough down those and you find the very idea of the Caveman's Cave - or perhaps you find yourself in the unformed versions of Babylon, of Ur, Assur or Akkad that never really took shape outside of the minds that built them.
Of course, this implies you could also go laterally. You could make endless subway stations spawn in, crisscrossing hallways right out of the Elementary School that Never Was. If the Endless IKEA gained traction, why not the Endless Supermarket? Why not the Forever DMV? Everything that defines our present time should also be logically present in the Backrooms, as compressed and stretched-out as everything else.
Imagining death traps and gauntlets feels a bit easy to me, especially when the concept seems tailor-made to speak of the horror of what is. A trauma ward you can't escape, a hostile office floor that never kills you but that finds ways to grind you down over time. Campus corridors that feel like they're straight out of their own rumored origins as rejected insane asylum hallways, or congested freeways that go nowhere.
The Backrooms should be where all the mundane fears of every former instant some civilization called its Present Time are made to roost and gestate, to turn into more primal forms of themselves.
At their most primitive, at the last possible mile that can be traveled on foot, the Backrooms should be an open and icy plain, with no enemies to speak of, no Hounds to chase you. Nothing except winds and ice, and the distant bellows of fleeing mammoths echoing up ahead. Either you stay in place and freeze or starve, or you find an abandoned spear and run, unaware of how likely you are to so much as catch up to your only remaining source of food, warmth and respite.
