Many airline passengers take off their shoes (I do take off my shoes most of the time too) while flying and sometimes their socks as well (I have never done this however).
Despite my years of flying, never have I seen this combined with putting your feet up on the seat in front of you as I witnessed yesterday on a LAN flight from Sao Paulo to Lima.
Flight attendants didn’t say anything, but I was left just shaking my head.
Some people appear to be checking out their brains when taking their seats on airplanes. Aircraft no matter which cabin you are in are essentially public spaces, so how someone can even remotely think that something like this would be acceptable is beyond me.