Markus Grandpré writes:
Hopefully we won't have to wait too long with Debian
Debian has traditionally been quite slow to update Mailman, and they're following tradition. Both the testing and unstable distributions are still on Mailman 3.3.8. Regarding your specific issue of Kein/None, testing has the same version of mailman-web (dated 2020-05-30), while unstable has a much newer version of mailman-web (dated 2024-03-12). That should resolve that issue, but you will have to wait a full Debian development cycle at least.
Our standard recommendation is to do a source installation in a Python virtual environment, point the new installation at your $SQLDB, review the content of /etc/mailman3 (I think Debian puts mailman3 configs there and that's what we recommend), test the new installation, and if that works, remove the Debian installation after preserving the /etc/logrotate.d/mailman*, /etc/cron.d/mailman*, and /usr/lib/systemd/system/mailman*.service files which are probably generally appropriate for Mailman 3 installed in a Debian system.
There is also a containerized distribution of Mailman 3 suite, but I don't know how straightforward it is to port over a Debian packaged system to that environment.
It may sound pretty fearsome, but you can hire people to do the installation and testing (maybe for as little as a pizza and a mug of beer ;-). After that you will get a more reliable system and better support (really, almost all the Reply Guys here run up-to-date systems of that kind, so we're much more familiar with it than with distro installations).
Steve