It's clear that a lot of real-world HTTP implementations are using HTTP 418 to mean I'm A Teapot, whether or not they should, and that reassigning it would cause practical difficulties.
If they like 418, they can add it to the spec. But this idea that it's been "consumed" is weird. It's like saying <blink> was "consumed" in HTML because two browsers used to support it. It was just an unofficial feature of popular vendor software, and is now officially disavowed.
Basically what we're saying is we're not going to implement 418 in the spec, but at the same time, we're not going to let anyone use it?
Which is accurate. Reintroducing a tag named "blink" into HTML to mean something other than "blinking text" would be a really bad idea for compatibility reasons.
Like I say, we could also just add it to the spec... But that would be ridiculous, because why would you support telling someone you were a teapot in an HTTP protocol, when the feature is only useful in a protocol for brewing coffee? That would be like taking a programming language's syntax for assigning data objects and turning it into it's own standard for a text file format.
Maybe a time traveler set this whole thing up 19 years ago just to troll people on standards boards.
Looking at you, JSON...
It's not like this RFC ratifying a 20-year-old joke that 'made it' is going to generate a cavalcade of lookalikes.
Some people used to think 65536 code points would be more than enough to encode every character in every language. Later it turned out not to be the case. Expanding that space necessitated the creation of hacks of various degrees of awfulness like UTF-16, CESU-8, WTF-8; today nearly every Unicode-aware environment has to be prepared to deal with unpaired surrogates somehow.
Expanding a fixed-size namespace is a pain. Who knows if HTTP status codes won't become scarce some day. They better not be wasted on frivolities.
You honestly can't, in this situation, be the judge of what is "frivolous use" and what is not.
Olive oil was used as lantern fuel before it was ingested.
Some swear by it, some want it banned.
Only the user can judge!
E.g. once (if ever) we're down to 10 or so free ones, assign them as broad categories and specify that the response text needs to contain an additional, longer status code (so, say, "499 1234 some text" and "499 4242 some text").
It's not as if tech doesn't have a long history of including extension fields like that, and it still works with the existing grammar