The new planet.archlinux.org has a different implementation and RSS feed url. I've added fallback urls for two variants of the RSS feed but would like users to switch to the new url so a news post seemed appropriate.
News proposal below:
# Planet Arch Linux migration
The software behind planet.archlinux.org was implemented in Python 2 and no longer maintained upstream. This functionality has now been implemented in archlinux.org's archweb backend which is actively maintained by offers a slightly different experience.
The most notable change is the offered feeds and the feed location. Archweb only offers an Atom feed which is located at https://archlinux.org/feeds/planet.