Hi Shug - there's never been much on-stage swearing at all in the Scottish folk clubs I've been to since the 1970s. It would certainly have been frowned upon by organisers at that time.
John Watt's story about the night Frank Sinatra played the Kelty Miner's Social Club [ which was hilarious ] - was the only single example of the c*** word I ever heard in all that time. "Bugger", "bastard" would occasionally turn up in the odd song, very few "f***s", although Dougie MacLean let one slip out at a big concert in The Aberdeen Music Hall, in front of the Lord Provost and his wife.
Good luck with the book.
Kenny