" There used to be those who insisted that it had to be a traditional song, sung by a traditional singer".." Dave the Gnome
I have never come across a club that insisted that it had to be a traditional song, sung by a traditional singer, you have made that up.
          There were a few clubs that insisted that it was traditional music only, they generally called themselves Traditional clubs, the policy was clear, Those clubs never excluded Revival singers from singing traditional songs and there was plenty of choice of other clubs with different polcies
The Singers club had a different policy again, but it was clear.
No one was obliged or forced to go to any of those clubs., there were blues clubs where blues was the order of the day
There is a club in Croydon which is a folk and blues club, and a different club called Croydon Folk Club, are you against a club that specialises in Blues? or is your dislike of policy or rules applicable to trad music clubs?