Btw Arkhan, not to nitpick but... the original DOOM didn't have slopes.
Uhh, what? I'm pretty sure I've seen Slopes in Doom, and not just staircases, and not just in the source ports.
Otherwise what you said about the engine is pretty accurate.
You see slopes in the logical, obvious, mathematical sense (lots of steep stairs = slope).
but from a goony technical hoojoo level building/environment standpoint, there are no actual slopes.
It has slopes in the textbook mathematical sense with Riemann sums, and with player travel. Otherwise, when you walk off a platform, you'd fall straight down.
It's nitpicking and depends how you are looking at it.