NSFL is just another name for gore or footage of death or injury. “Not safe for life” is not a thing. There’s no actual image that can kill you unless you have epilepsy or some other form of photosensitivity. I heard “Not safe for looking” but that’s an idiotic acronym, and most people don’t interpret NSFL that way at first, and it still has the connotation that it can physically harm you for some reason.