I glue in frets. even if they're in with CA, they can be removed in most cases by putting an iron on the fret briefly.
I dot the very ends of the fret slots with thin CA - If someone would like to boast about their never glued frets and then have a fret end lift and have to deal with it later - they'll probably end up gluing it.
I saw a well-respected custom shop in the US (figuring they'd have some ultra precise setup where they wouldn't need to glue in) setting frets totally by hand and very carefully beading the entire slot with CA. The comments on the video were filled with "tycoons" who probably never worked on a guitar. I'd bet the shop itself just didn't feel like dealing with come-back guitars with
I use hot hide glue for frets. It helps them slide in, and when set it fills any voids around the tangs. Fish glue would do just the same.
One concern with fish glue is reports that it can soften in high humidity - for guitars we're talking Alabama or playing in the bathroom. I suspect that it's only a problem with very thin wood, as it takes ages for moisture to percolate naturally into thicker pieces.
I've not come across that problem myself. I follow Tom Woodford on YouTube (Twoodford) and I got the Fish glue idea from him for fretting, used to use CA, both are great, and both can be broken down with heat as D_W states, don't think I've the patience for Hide Glue and fretting