This proposal can serve well to standardize the header itself, thats
the easy part. But how it will apply is a different ball of wax. The
MUA side can deal with it by moving it into folders or deleting it.
...or by showing them in a different color, or optionally hiding them in
summaries. if the only options most MUAs provided were effectively
deleting them, that would IMHO be a bad result.
Again, we could explicitly specify that this MUST NOT be done,
and include proper reasoning for this decision. One would be
stupid to implement a MUA which still does that then.
Cheers,
Michael