On the "cruelty scale", that'd be a "Cruel", since it has a "walking dead" state of "you did it wrong a while back and have to start over":
https://eblong.com/zarf/essays/cruelty.html (and https://eblong.com/zarf/essays/cruelty-revisited.html )
Early King's Quest games were in that same category of "Cruel", where an incorrect choice is not immediately obvious as a mistake.
I think King's Quest V was closer to "Nasty" (because it can catch you off guard with things you fail, but it'll be obvious that you did something wrong). But there are a couple of exceptions that keep it in "Cruel", though: you can eat the wrong item when you're starving, and in some cases if you miss picking up an item you can't go back for it, so if you didn't see it at all then you have to go back to an earlier save. That one definitely required keeping a full array of save files. (Very satisfying to complete, though.)
Even King's Quest VI, which is much more forgiving, has some cases of "you need the right item before proceeding into this section you won't be able to leave (for a while)". But it goes out of its way to advertise those "points of no return", so I'd rate it firmly at "Nasty" rather than "Cruel", with the asterisk that there are ~3 notable points where you might have to go back by more than just your most recent action, but it'll be obvious you want save points at those points to go back to if you find you can't progress inside. (Before the catacombs, before the land of the dead, before entering the castle.) The entry to the catacombs even offers you the chance to go prepare further, if and only if you don't have everything you need.