Judging by r/youtube, they've been going down about every month this year. Feeds were down for some hours a couple of days ago, too. It might be temporary, but we can't tell without an annoucement.
It seems to be working again. Most likely they were down temporarily for whatever reason.
Until very recently, people could subscribe to a YouTube channel (or playlist) with a URL like:
The title link was previously the feed for The Late Show. In their continued war on everyone, Google appears to have killed YouTube RSS feeds as these all now return 404.