Mailman used to be standard some decades ago, but fresh users don't know how to work with it. Other software-as-a-service providers try to assimilate users on their own account, e.g. stackoverflow.
Various high-end projects now have a self-hosted instance of https://discourse.org to make the best out of old and new traditions.
Is it feasible to use it as a replacement for Mailman mailing lists, issue trackers, and fancy games like zulip at the same time?
See also https://matrix.org "An open network for secure, decentralized communication"
Zulipchat (https://zulipchat.com/) is a chat service that has an interactive eMail-like threading model. Content is formatted in markdown complete with code formatting and more. With its filter mechanisms, one can easily concentrate on single threads or mute threads that one is no longer interested in. Since Zulipchat is open-source, it is possible to self-host it and use a variety of authentication schemes (https://zulip.readthedocs.io/en/stable/production/authentication-methods.html).
The Lean community is successfully using Zulipchat and users of Lean cited the accessibility of the chat rooom as a reason for deciding to use Lean in the first place (https://leanprover-community.github.io/papers/mathlib-paper.pdf).
With the current situation leading to increased activity at https://isabelle.zulipchat.com, it would be a good opportunity to discuss a switch to Zulipchat on the mailing list some time after the Isabelle2020 release.
We can certainly discuss self-hosting of the Zulip platform (or alternatives), but I personally don't want to be in a "cool place" at all.
Flarum https://flarum.org looks like a pretty good alternative to Discourse, Matrix, Zulip etc.
After a few years of development, it has reached stable 1.0.0 in May 2021.