They all seem to be still available in the Wayback Machine (for example: Gmane -- Mail To News And Back Again), so perhaps all these links should be redirected there.
Great suggestion, I’ll do that.
Thanks @calion for pointing that out… we always welcome pull requests as well!
Hm. There is nothing on that page that indicates that pull requests for it are possible.
Nor is a PR needed…I had already modified spec.txt to use the archived links. This isn’t reflected in the most recent version, but it will be in the next update of the spec.