YouTube RSS Feed URIs
These appear to still work (for the time being), but it's clear that YouTube/Google/Alphabet want to transition to a world where URIs are just an endpoint where you present your API key, the better to surveil you and your users:
https://www.youtube.com/feeds/videos.xml?channel_id=CHANNELID
https://www.youtube.com/feeds/videos.xml?user=USERNAME
https://www.youtube.com/feeds/videos.xml?playlist_id=YOUR_YOUTUBE_PLAYLIST_NUMBER