I personally don't like smut but when I use the 18+ tag for a roleplay, it's my way of giving a heads up to anyone who is reading the thread that there's going to be mature themes like strong language, violence, and usually, graphic descriptions of gore, but that's about it.
I have noticed that there's a lot of people looking for smut more than anything else in the 1x1 Interest Check section though, but if you're lucky, you'll find some people who aren't.