Jakob writes "Mailman is intentionally disregarding the RFC and causing its default behavior to cause problems for postmasters whose users are permitted to participate in mailing lists ..."
I assume the RFC you are referring to is RFC 7960. I have read this RFC and I do not draw the same conclusion. In particular, section 2, paragraph 2 says
Note that domains that assert a "p=none" policy and email messages that pass standard DMARC validation do not have any interoperability issues.
which implies to me that it is not necessary to apply any mitigations to messages From: domains publishing p=none.
Clearly you believe otherwise. Please identify specific RFC sections and language that support your opinion.
The mitigations of modifying the From: header or wrapping the message have negative consequences for users of at least some MUAs, so list owners have incentives to apply them to as few messages as possible. We are currently working on the mitigation strategy for Mailman 3 and have decided at this point to not have controls over which policies trigger mitigation, but when mitigating based on policy to apply mitigation for p=reject and p=quarantine, and not p=none. If we are convinced that RFCs say we SHOULD or MUST apply munge/wrap mitigations to messages from p=none domains, we will do that, so please tell us why you think they do.