I was on SFO-FRA (UA 58) yesterday, which got delayed by over an hour because they forgot to clean the plane. (I'm not sure if they actually forgot to clean the plane, but the cleaning crew was MIA at boarding time even though the inbound aircraft had landed 4 hours prior.) When we finally boarded, there were no slippers on our seats. The FA's checked, and no slippers had been boarded. One of the FA's insisted that slippers are only guaranteed on flights of 12 hours or longer. I seem to recall that it's only the pajamas that have the 12-hour length requirement, and that slippers are supposed to be on every seat for every Polaris flight.
I'm not going to complain, but if I get a survey from UA I'm going to mention this. I just want to make sure that I'm right and the FA was wrong, so would appreciate the confirmation.