According to Goeff Nunberg on Fresh Air yesterday, the swearing on Deadwood is anachronistic. Nobody said fucking and cocksucker all that much in the 1870s. These didn’t get popular until the early 20th century. For serious shock value, they would have used true profanity as opposed to obscenity, that is, blasphemous expressions like god damn it and hell and jesus christ. You can see why the writers decided to crank it up for modern audiences, whether you agree with the approach or not.
And here I thought the show’s every detail was historically impeccable. Oh the disappointment.