420 is not a standard error code. It was used by twitter back in the day but was replaced with 429 when that became standardized. In fact due to this non standard usage and the conservative nature of standards bodies 420 will likely never be a standard HTTP response and will never be as good as 404.

You're completely right about 418.

Company: we need advanced javascript and bloat for paywall

HTTP: 402 Payment Required

Just https://http.cat/, no www.

451 is the third.

+1 for http.cat reference.

I'm always happy to see a HTCPCP reference!