Don't get confused by the name of the extension that it's somehow SMTP-only.
> And there is standard way to encode any charset in (almost any) message header for ages.
Actually, no. For example there is no allowed/standardised way to encode a MIME From address that uses UTF-8. It is only permitted to encode the non-structured part of a header such as the comment or phrase, but not the structured part (the address itself). It is also explicitly forbidden to encode anything in the Received header field.