SHOULD is a recommendation, not a requirement, and it refers only to newly-defined header fields, not existing ones. The text implies that 8-bit characters in existing fields are to be interpreted as ISO-8859-1.
There is a RFC (2119) that specifies what SHOULD means in RFCs:
> SHOULD This word, or the adjective "RECOMMENDED", mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course.
> SHOULD This word, or the adjective "RECOMMENDED", mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course.
https://datatracker.ietf.org/doc/html/rfc2119