I have come to the conclusion that it is very much down to the club organiser(s). The the repertoire of the organiser is primarily traditional songs then he/she will tend to attract like minded people and this then provides a core of regulars who sing mainly traditional songs. This will then set the tone for the club and it will gradually get known as one where mainly traditional songs are sung.

This is my observation from the clubs I attend locally both those where I go regularly and those where I go or have gone occasionally.

If you can set the tone in this way, you do not really need to have any formal rules about what is expected in the club.