Earlier quoted context omitted.
So we're trying to avoid breaking the internet by committing to people expecting an error when they're talking to a teapot? So that past implementations of clients connecting to teapots won't break? 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 bre…
It's a miniscule bit of humour that consumes 1 of 31 used slots in a 100-slot block, in an area of tech that isn't even remotely drowning in 'fun stuff'. What does it really matter? There are plenty of perfectly serious RFCs that are even less implemented than 418. 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.