Actually, there was a proposal to remove the 418 code formally, but in the end it was grandfathered in. Unfortunately, unless you have convinced a lot of people to allow 419, it would be not allowed anymore (even in a April Fools' RFC) according to the established protocol of IANA controlling the allocation of error codes, and IANA no longer allow "joke" allocations unless there was an RFC clarifying why that particular code must exists in a non-joking manner (see 451, in homage to Fahrenheit 451 but is the recommended code for a informed block). Even 418 was technically only reserved in such a way that allows it to be overridden in case that a good demonstration that 418 should be the code for that error.