Yes:

Unfortunately most quest-related NPCs are essential even when it's some stupid little quest that doesn't matter and should really just be failed by their death instead because it's literally the least important thing ever. In fact I think you can see one of those in the background: there's someone on their hands and knees and I think that's someone with a short quest that goes nowhere, but who is still invulnerable.

Edit: it's not every named NPC, like I got at least one of those while clearing the area for that screenshot, and at least sometimes the flag gets cleared once the quest is done.