As best I can tell from reviewing the documents, Sensitivity
(and Importance) were defined first by MIXER in RFC 987.
I believe that's correct.
VPIM then grabbed ahold of the same fields in RFC 1911.
When MIXER was revised as RFC 2156, there was a note
cautioning about other (non-MIXER) uses of these fields.
RFC 1911 was then updated to RFC 2421 (and again to
3801), apparently ignoring the MIXER cautionary note.
I would like to think that MIXER is dead for all practical purposes. There
certainly is little if any chance of the specification ever getting updated
again. I therefore cannot see taking this cautionary note very seriously.
However, I should add that only a few months ago I was contacted by someone
writing a brand new implementation. The mind boggles...