OK, it was this one. https://refeed.to/feeds/CIipRO1b.xml When I pull it up in an RSS extension it doesn't come up either, just the XML comes up.
- https://feedly.com
- https://www.inoreader.com
- https://www.newsblur.com
- Feedbro, a Firefox extension
- https://theoldreader.com (mentioned by you)
- elfeed (an Emacs package, so not web-based)