A floor's actual stairwell cannot be destroyed, inventoried, moved into a saferoom, or removed from play.
Milk records that platforms supporting stairwells may sometimes move, but the portal itself cannot enter inventory or a saferoom. One stairwell survives a direct Utter Annihilation wand strike without damage.
Herot confirms the formal rule: a stairwell may remain gated until conditions are satisfied, but cannot be removed from the playing field. This explains why submarine destruction and other extreme events did not destroy prior stairs.
The protection does not extend to the magical doorway or chamber access surrounding the portal.
A glowing staircase untouched inside a blast that annihilates everything around it.
The rule guarantees that the floor-to-floor mechanism persists even when every practical route into it is destroyed.