YouTube still does

    <feed xmlns:yt="http://www.youtube.com/xml/schemas/2015" xmlns:media="http://search.yahoo.com/mrss/" xmlns="http://www.w3.org/2005/Atom">

I don't think they are linked to anywhere but the url is http://www.youtube.com/feeds/videos.xml?channel_id=<channel_id>

Google on several occasions took moves to make the web less semantic.

They dumped microformats and standards in favor of soupy error tolerant formats that benefitted their search engine and made it harder for other efforts to make information shareable and accessible.

They wanted it to be easy to get information in, but for you to have to go through them to get information out.