There are many threads about this already, but they’re all 4 years old and reported to be fixed. However, I still cannot add https://what-if.xkcd.com/feed.atom to NewsBlur; I get this error:
There was a problem trying to add this site. Please try a different URL.
This despite the feed being valid: https://validator.w3.org/feed/check.cgi?url=http%3A%2F%2Fwhat-if.xkcd.com%2Ffeed.atom
It seems to have been broken for a while; I think Niagara Straw was the last one I have seen.
Otherwise, keep up the good work!