Some pilots used the term SLF for passengers, especially those with a military background, as used to be the norm:
I arrived into Airline flying in 1982, and the aged RAF gentleman around me would use this phrase. A phrase I had never heard before. In the intervening years from Dan Air, Britannia, and BA I have
only ever heard passengers referred to as SLF by pilots. And then only rarely by military pilots. Indeed a passenger is just that in the military. They are not paying customers. They are exactly what it says. Self Loading Freight.
I have never ever heard a Cabin Crew member in any airline that I have worked for call passengers SLF.
I think it is worth remembering that this is historically a Pilots forum. And an old one at that. Unusually for the internet it carries with it history.