1. Go to the YouTube page for the channel.
2. View the source for the page.
3. Search for 'channel-external-id'. Copy the string for that attribute, e.g. UC5nVgiv6elz5GnS6MTr1tVA
4. Create a URL with the channel_id, like this:
https://www.youtube.com/feeds/videos.xml?channel_id=UC5nVgiv6elz5GnS6MTr1tVA
5. Use that URL to subscribe in your favorite RSS news reader.
From: How to Get an RSS Feed for a YouTube Channel
