Really? Several times I thought I saw threads mysteriously being ranked downwards, only to find out they contained a formal (or other) discussion of swear words, so I always concluded that there's some sort of "shitstorm detector" at play. But maybe I'm just imagining things...

Your observations were probably real but you might have misinterpreted them. We do penalize subthreads for being shitstorms, flamewars, or off-topic. (Tediously off-topic, that is. Whimsmical offtopicness is fine.) And profanity is more common when people are upset, so one would expect a correlation. But profanity isn't a moderation concern per se.