...
I don't discount this possibility, but the rejections just go back to the Mailman server and the list, and reports of the rejections go to the people publishing the DMARC p=reject policy for their domain. I don't see how any of this winds up being delivered to some third party's spam trap address.
I.e., the people (or bots) at Yahoo receiving reports of your DMARC failures might decide to take some action against your server, but that would be Yahoo blacklisting you for excessive DMARC failures "impersonating" their domain. It wouldn't be maps saying you're sending mail to their spam traps.
But since the OP said ". New subscribers are asked to subscribe to a yahoo group instead."
I assumed it was really Yahoo (perhaps under a mask as mail provider for some other named service, ala comcast ;>) who was doing the blacklisting... who else would be recommending yahoo groups as an alternative?