Those ain't slopes, Arkhan... those are just height variations. By your definition, a staircase is a slope (that's all DOOM has that is slope-like). You could, in theory, create a number of tiny sectors that are 4 units apart and 1 or 2 units tall and call it a "slope", though this wouldn't work too hot on the original DOOM engine as it would crash pretty damn quick. Again, slopes were not added until ZDOOM. My own PWAD (which is based on Heretic) uses slopes here and there, my favorite GZDOOM PWAD (The Ultimate Torment And Torture) uses them extensively. Neither of these will even load in the original engine, let alone run.