Every dungeon needs a broom closet.

I was looking at a screenshot on Steam with a caption that a dungeon was too messy with dropped loots. So I was thinking there should be little rooms off to side of passages with signage that this was were trash belonged. It has the advantage that you know if something that is in there it is unwanted and you can pick it up. It is not someone's art project or inventory management system. And the drops don't clutter up the hallways. Closets, boot rooms, cloak rooms, jail cells, and probably some other types of rooms would work. There is some sense to locations having these kinds of rooms. Players may ignore these rooms and still just drop stuff everywhere.

Another idea is that maybe there could be bottomless holes in dungeons that act like trash cans, and maybe there could be a rare chance that a particular drop would result in something more interesting being spit back out of the hole.

Cleaning slimes, rats, and flames that can be fed or that automatically roam around grabbing and running off with or destroying dropped items are another method.

or maybe a debuff called loot blindness, if you drop a certain number of an item till the the debuff times out you don't see that kind of item in loot window anymore, this would make players more careful about being neat I think.