The thing that really disappoints me is that 418 I'm a Teapot isn’t registered—instead it’s reserved as “(Unused)”: https://www.iana.org/assignments/http-status-codes/http-stat..., https://www.ietf.org/archive/id/draft-ietf-httpbis-semantics.... As it stands, I suspect (as one that’s been involved in a couple and examined more back in 2013–2014) that most even vaguely recent HTTP libraries that have some kind of status code enum or constants defined take their data from the HTTP status codes registry, with a single exception for 418 I'm a Teapot.
As far as a 419 is concerned, I’d argue that 418 is already suitable anyway as a joke alternative to the more serious 429/503: “wp-admin.php? I’m not WordPress, I’m a teapot!” (Similar style to the joke about one cow warning another about the mad cow disease in the area, and the other responding that it’s not not worried because it’s a helicopter.)
418 is defined in HTCPCP, an _extension_ to HTTP. That's why I never understood why people use it in HTTP. So it makes sense that it's only reserved in HTTP.
As far as a 419 is concerned, I’d argue that 418 is already suitable anyway as a joke alternative to the more serious 429/503: “wp-admin.php? I’m not WordPress, I’m a teapot!” (Similar style to the joke about one cow warning another about the mad cow disease in the area, and the other responding that it’s not not worried because it’s a helicopter.)