Just tried to subscribe to the RSS feed for https://apod.nasa.gov/apod/astropix.html. Huh, seems that I can't. Why? Because they're sitting behind Cloudflare, and Cloudflare demands that you prove that you're human in order to subscribe to it. As my feedreader isn't human it struggles somewhat here.
Turns out this has been going on for a while and it's impacting a lot of sites and open standards software.
https://openrss.org/blog/using-cloudflare-on-your-website-could-be-blocking-rss-users