IMHO, feet (clean, dirty, shod, bare) never belong on the 'furniture'. That includes on a plane.

I do kind of wonder about the people who get grossed out about someone going into the toilet in socks or bare feet. The disgusting grot they pick up on their feet is no different than what gets picked up on the bottom of everyone's shoes - and tracked all over the cabin, including the area under the seat where you stow your bag - the bag that you might then pick up and set on your lap or your tray table.