I think you need to be careful to not confuse height and cover.
When you have the sun in your back and are on top of a building hiding behind a small wall on the roof, like a sniper would do, then you are hard to spot (and to hit). But that is not due to height (alone), but mostly because of the other factors mentioned. So mostly you can attribute that to cover, not height.
Look at most combat situations in BG3. Someone is standing at the top of stairs (2m high, 5m away), the other person at the bottom. Both persons see the other in full view. Now imagine both aiming at each other. Why would have the person on top of the stairs have more than double higher chance of hitting then the person on the lower ground?
Height is already an advantage in and of itself - melee fighters will have a hard time getting to you, most of the time.
So no need - imho - to give characters/npcs the DnD gameplay rule/mechanic called "Advantage". Because a ranged attacker does already have the advantage of being in an elevated position.
