The Kelman "variants" include "fucking," "fucked" etc....

To address your original question, I don't know of any folk clubs or festivals that have any prescriptive rules re: strong language (I'm in Australia BTW). In my experience, it's not ever been overdone, with one exception - as noted above, Steve Earle. Started to watch his set at the Port Fairy Folk Festival a couple of years ago, and after five minutes of his stage patter sounding like every second word was "fuck," I gave up. His singing was equally as boring.