I disagree so much with Jim Carroll's posts that maybe I'll start by saying that MacColl was an excellent songwriter and that he certainly was not against contemporary songs, in the 'tradition' as defined by himself and the Critics Group- Ed Pickford, for one, was encouraged by by him and Peggy Seeger. I visited the Singers' Club a few times in the sixties, and recall the restrictions were political rather than anything else. It'd have been a brave singer who sang or said anything in favour of the apartheid regime at such a time!

Other clubs applied restrictions of a non-political kind- Louis Killen and Johnny Handle emerged from the jazz/skiffle scene in Newcastle and when the new Bridge club was set up in 1958 they decided that only UK/Irish material was allowed. This choice was entirely justifiable in its time- remember this wasn't long after a time when Seamus Ennis had been commissioned by RTE to head off to the West of Ireland on his rusty bicycle and 'collect all he could before the next wave from the Atlantic washed it all away'.

Re material performed in folk clubs, I can only repeat that even if Jim C knows what is appropriate in such places and can even define 'folk' and pop music, I would contend that such tunnel vision is nonsense. I may well be already sought by the folk police for singing songs by such as Cooke and Homer & Guy Mitchell (last of the ballad singers) so I must be another very bad person?

September Song is a wonderful composition and for me is quite acceptable in a folk club. However an acceptable performance may be a lot more difficult to achieve by your average floor singer than by a quality singer like Frank Sinatra's.
I have a lovely memory of one night at the 'Empress of Russia' folk club of a song performed by Nick Havell, trombone player with Flowers and Frolics at that excellent club in the 70s. Nick was a great opera fan, unlike me, and his version of 'La Donna e mobile' backed by the Flowers band will live long in my memory. (this is relevant to the thread, incidentally!).
So you'll gather that while I accept there are vague genres of music like folk, pop. jazz, classical, trad if you insist, and opera , I would say that the performance is everything , and that blinkered views of the 'tradition' as defined by such as Jim C may be one reason why the UK folk clubs are now so reduced in number. Mind you, in my experience, mind you, today's clubs seem a lot more tolerant than that.