Well, it's all depends on what you mean by folk. Personally, I would have thought any workable definition of folk would have to include the vernacular, unpublished, word-of-mouth disseminated type of thing. And "unpublished", naturally, will always include "unpublishable".
Having said that, there is an argument for not using the more common playground insult words in the titles of threads, if they cause offense.But that argument is only on grounds of limiting potential readership. There can be no argument for completely excluding songs because they contain swear words: unless you want to designate Mudcat as an International Forum for the Discussion of Polite Folk Songs?