I would like to suggest this for a reopen: https://math.stackexchange.com/questions/3825154/proof-that-set-is-not-a-polyhedron?noredirect=1#comment7891303_3825154 I may be missing something obvious, but personally do not think it trivial.
@SmokeDetector OP vandalized their question instead of improving it. That has been reverted, but since further improvements of the question cannot be expected it can just be deleted.
