What a lot of fuss. I don't know where the phrase SLF came from, but in over 30+ years on the flight deck with a mixture from military and civilian background I have never heard the term SLF, until I read PPrune.
I don't think it is in common use, maybe this thread has made it appear more common than it actally is.
If it is a term used by cabin crew they have never used it in my earshot.
Pax, passengers is always used, in my experience.