NoTrim is a TEXTURES flag and an ANIMDEFS keyword that was added following my suggestion.
It doesn't seem to work at all. I tried it yesterday on g4.8pre-321-gecc0afd2f.
If specified in TEXTURES it produces no visible effect, and if I try to add it via ANIMDEFS (which should be possible according to this https://zdoom.org/wiki/ANIMDEFS#Other), it actually produces a "bad syntax" error, so it doesn't seem to be recognized at all.
I don't feel an example is particularly necessary but I made one anyway.
TEXTURES issue: I made a pistol version with PISGF0 sprite that has empty canvas twice as tall as the sprite itself. It's continuously rotated at the center, which should be the iron sight given the empty canvas, but it's rotated around the graphic's center instead.
ANIMDEFS issue: uncomment the "Notrim PISGF0" line in ANIMDEFS, and GZDoom won't start.