I think that inescapable death pits have no place in classic Doom because they kill player too slowly. It's very frustrating to get fall down and just wait for sweet release of death. However insta-kill pits are okay because they don't fool the player that maybe he can still escape and of course dying is less frustrating than waiting for death.
So if you want to kill player for falling down don't waste his time and do it quickly. I think it's pretty obvious because every other game use insta-killing pits