I cannot understand the reason behind asking such a question, other than idle curiosity.

If any performers have suffered discrimination because of this, it is very likely unlawful. Even if it is not actually illegal, it is unlikely that any folk club organisers are likely to admit to it in public, and certainly not in this forum since it is probable that most Mudcatters would find such discrimination unacceptable. Anyone else's opinion, apart from the musicians themselves, is simply speculation.

If Dick has good reasons for asking this question, then he should contact those performers privately. If this uncovers evidence of discrimination then that might be a valid subject for discussion. I don't think the question Dick asked is.