My partner and I did use a safeword for a while in non-kink situations after I was assaulted. I'm someone who says "no" to mean "not there" or "that's too hard" or "I don't want to do that; I want to do this instead", and we were both worried that "No, this is triggering me" might get confused with all of that. Hence, the safeword.

Before that though, I had only associated them with D/s situations.