I’ve been poking around what Discourse can do, and it appears there is a plugin that will mirror newsgroups: https://github.com/sman591/discourse-nntp-bridge The last commit is a bit old, but maybe it just doesn’t need anything new?
It looks like you can easily import mailing list archives if they are in mbox format. https://meta.discourse.org/t/import-mailman-archives-into-discourse/18537/11
Consolidation of content. With all the various discussions around Thunderbird happening in multiple locations, it’s hard to get a good feel for what is actually going on. Mirroring the newsgroups and mailing lists onto Discourse fixes that. It will be especially helpful for anyone who starts looking for help on Discourse first. They will be able to search all the archives at once, instead of having to find the Google Groups mirror (if it exists) to search. (Or download all the messages from the list…)
Beyond that, I would also encourage moving away from mailman and entirely onto Discourse. Discourse can emulate the essential features of mailman that fans of mailmain like (at least I think, I am not a mailman fan…), while also offering a much friendlier entrance for new users.
Migrating would also avoid having to figure out how to enable two way mirroring between systems. We’d just have to figure out how to import all the archives.
Anyway, what does everyone else think?