Hi, I had a friend who last year decided to leave SL permanently and delete her account. She had some objects rezzed on my land and as long as I owned the land her stuff was still there, before she left SL she gave me mod rights on her stuff so the 2 or 3 items she left (which were transfer) I was able to take with me when I moved. So the simple answer is YES their objects remain in SL. If there is an item that has to be returned to the deleted account it gets deleted, but as long as the objects are in world they will remain until taken by someone who has mod rights (if transferable) or returned to the deleted account.