I'm mostly seeing it on kinkmemes, which aren't enforceable in the same way as an exchange, and "DNW: anything yucky" kind of makes sense if your prompt is for, like, flower crowns in a field of fluffy bunnies. But I have definitely seen it on prompts where "general bad stuff" makes 0 sense as a DNW, even for an informal prompt collection. And I dread that it will start showing up in, like, yuletide, which you are right, would just. not.