http.cat is surprisingly useful because of the short url. Typing in e.g. http.cat/422 beats any other method I know for quickly looking up status codes by number.
Just type in "error 422" and a Google snippet tells you what it is. Saves 3 characters, doesn't depend on a single website, and returns a more descriptive paragraph instead of some silly image with no relevance...
I just entered "error 422" into DDG and got a useful result so if you interpret the parent post as "use your favorite search engine" then it doesn't depend on a single website.
And for something non-critical I would rather depend on a quirky independent website than on google. I want the internet to be more than six companies dominating everything.
It's listed, but under geographic TLDs[1] because it's Catalonia's TLD.
Interestingly this TLD only allows website that "to serve the needs of the Catalan Linguistic and Cultural Community on the Internet"[2] which is why http.cat also offers a Catalan version[3].