Both the OP's URL and the one linked by @xnx work for me, could it have been temporary? Or maybe a caching issue? (And my region's YouTube CDN servers aren't affected?) Could be Google are actually removing this but A/B testing it first?
For posterity, if it ever breaks I made a rough proxy script:
https://gist.github.com/yunruse/93dd40719568dccccebcca26d9ed...
I think I'm still going to look at building a yt-dlp/JSON to RSS alternative, though. I don't trust that this isn't foreshadowing and not a bug.
Not looking forward to the day they don't come back, of course!