I've mucked around with the RSS feed so that it validates again. I don't know how long it hasn't been validating. Seems like forever, since I'm using the kirby3-feed plugin and haven't changed it. I opened it today so I could add the Reply by Email link into feed items, which is when I noticed it wasn't validating due to a non-full URL in the item->link element. I had to hardcode "https://baty.net" into the link, which is a total hack, but at least it validates.