The RSS feeds are returning 404, for example https://www.youtube.com/feeds/videos.xml?channel_id=UCEIwxahdLz7bap-VDs9h35A and link that used to be on the channel's "Videos" page is gone as well.
This is not the first time this has happened. See
https://news.ycombinator.com/item?id=39179446
This is only based on a few-hundred requests because the service uses WebSub for YouTube so fetches primary occur when a video is published or updated.