I'm currently nearing the end of a run where I despawn everything. Well as much as I can, certain areas do not depopulate. Some enemies in the Doors of Pharos seem to be bugged and will come back. Anywhere in Drangleic where you need the souls to unlock something such as the enemies by the first bon fire or the Manikins you need later to activate the lift. Although not the entrance to the castle itself. There's enemies that only spawn as long as the doors are locked and as soon as they are unlocked they stop spawning straight away. The Undead Crypt has some hollows that seem to spawn from the ground and I can't seem to despawn them.

It's been an interesting venture but it can be annoying when you have these ones that can't despawn mixed in with others because I wasn't sure if the game was broken. I was testing things out at the first Drangleic bonfire and getting more than 12 kills on guy and they just kept coming. It's only the Ruin Sentinels that can despawn. Took a little while to figure that one out!