fermium, the server running mailman is currently running jessie. There's the wider question wrt Mailman 3, but even if that doesn't happen yet, Mailman 2.1 continues to be available in Stretch and Buster, so the updates of the OS and Mailman version would not need to be coupled.
lists.wikimedia.org is now running from the buster host lists1001.wikimedia.org.
Fermium (the old lists host) has been shut down (via gnt-instance shutdown) and removed from puppet (remoted cert and from puppetdb). I'll leave the VM data itself alone for now, just in case.
Transitioning this to resolved, but please don't hesitate to re-open if any issue are observed.