A new site has a feed here: X-Plane.to | X-Plane 12 Mods, Liveries and Add-Ons
And the error:
This address does not point to an RSS feed or a website with an RSS feed.
The feed was invalid, but they fixed it 12-24 hours ago (see validation here). I and others saw this error in 2016, I don’t know how to diagnose. Not sure if it’s cached or blocked or what.