In general we have to think about how we are going to replace
mailman2. It is based on python2. Both aren't maintained anymore. We
are doing fine because lots of others organizations are also still use it.
And we managed to get all the changes we need into the RHEL mailman2
package. So it will still be supported for some time.
 
There isn't yet a direct replacement with all the same features.
The most likely alternatives are mailman2 and mlmmj.

mailman3 https://list.org/ seems more a user forum (with django account management) than a mailinglist. The archiver is pluggable.

mlmmj seems under active development, but you have to know where
to look https://codeberg.org/mlmmj/mlmmj/issues/3

We need a prototype setup and a migration path. An idea which
features can and cannot be supported. And confidence that future
email "issues" like dkim, dmarc, single-click unsubscribe, arc,
etc. can and will be supported.

inbox.sourceware.org seems a superior mailinglist archive setup.
But there might be things missing that pipermail offers?
Maybe it needs a pipermail http like "skin"?