RFC 2616 OCTETs are defined as "<any 8-bit sequence of data>" quote unquote, nothing is said about their value beign opaque.

      TEXT           = <any OCTET except CTLs,
                        but including LWS>

IETF rewrote the productions not to use TEXT, but stopped short from banning the old behaviour.

So, for instance, where 2616 states: Reason-Phrase = <TEXT, excluding CR, LF> And 7230 has: reason-phrase = ( HTAB / SP / VCHAR / obs-text )

It is making sure that any application that conforms to 2616 still conforms to 7230 by not making it illegal (MUST) to parse obs-text... Just something you SHOULD not not do. They are simply making it so any new header added is defined as SP / VCHAR only (quoted, possibly).