Thanks for the heads-up. I have to say I would have appreciated a little more advance warning time than one day. I spent several hours yesterday manually migrating ~150 old lists.swift.org links to forums.swift.org, both on my public website and in my private notes. And I was lucky to notice this post in time.
I'm wondering if there is any chance to preserve existing URLs to lists.swift.org. We don't need an active Mailman installation, of course, but a static copy of the mailing list archives that preserves the existing URLs would be really useful. (Or a redirect to the new links at forums.swift.org, though I assume this would be difficult.)
According to my GitHub search, the apple/swift-evolution repository alone contains 196 links to lists.swift.org posts. These links often provide important context to old SE proposals (e.g. acceptance/rejection rationales, or links to the discussion threads).
And as far as I know, there's no easy way to derive the corresponding forums.swift.org URL from a lists.swift.org URL, so many (most?) of these links will be really hard to fix once the maling list archives are taken down.
I don't know how Mailman stores its archives. If the archives are already a bunch of static HTML pages, have you considered keeping just the archives online? If Mailman generates the archives dynamically, have you considered creating a mirror (using
wget or something) of the archive pages and hosting the mirror at the existing URLs?