You CAN route a document deposited in the Drop off library, to a document set. You wouldn’t think so considering there is no way of selecting a document set within the browse-library UI, available in the Content Organiser rules page, but there is a trick. Just type the Document set name on the end of your library URL. Don’t copy and paste from the address bar- it won’t work. Make sure, if your document set name is multiple words e.g. Shipping ABC, then use %20 for spaces – /Shipping%20ABC
Ok so now we have that out of the way, I have a complaint about the Drop off library. Even if all metadata is filled out on a “dropped document” the document won’t be routed until it’s properties are edited. That seems completely pointless! I email enabled my drop off library will all metadata fields having defaults. When a document arrived it didn’t route until I edited the properties (which were all already filled out and correct) and clicked submit.