I think stuff dropped in out of the way area that been hasn't built on disappears after a certain amount of time. I'm not sure of the distance, I think stuff around buildings/bases stays around forever, or much longer.
It would be nice if the command to cleanup stuff worked on servers, I was considering just doing it the hard way, i.e once a week, or whatever, just download the world files and load in solo, wipe items, then upload the world, everything clean. people would need to pickup stuff, so a set "maintenance" prob best so people expect it.
I haven't been on, so if this a trick I need to do once in awhile, let me know.