This is very useful, thank you for sharing.

Doesn't seem to work for any of the ~50 URLs I tried with. Eg:

https://www.youtube.com/feeds/videos.xml?channel_id=UCS0N5ba...

... works but:

https://www.youtube.com/feeds/videos.xml?channel_id=UULFS0N5...

... is 404.

I'm guessing it's only a feature of playlist URLs which that SO answer is about, not RSS feed URLs.

Sorry, my notes were lacking. Change UC to UULF and use it with this URL to get the Videos feed for a channel:

https://www.youtube.com/feeds/videos.xml?playlist_id=UULFS0N...