They left off RFC-6919 [0]
The key words "MUST (BUT WE KNOW YOU WON'T)", "SHOULD CONSIDER", and "REALLY SHOULD NOT" are to be interpreted as described in RFC-6919There is also the RFC for the 7xx HTTP status codes [1]
[0] - https://datatracker.ietf.org/doc/html/rfc6919
[1] - https://github.com/joho/7XX-rfc [language warning]