Or does the DNT header predate MDN?
More generally, there are plenty of ad-hoc read receipt headers in use,
Return-receipt-to: (most systems treat this as a delivery receipt request,
but not all)
Return-receipt-requested: (same issues as return-receipt-to:)
Registered-mail-reply-requested-by: (my personal favorite)
Although the RECEIPT WG could have reused some previously employed ad-hoc
field, we very deliberately chose to avoid existing fields on the grounds that
the semantics we wanted were quite different than any previous field was given.
We wanted a new field nobody had used before so we could get the semantics
right. And we were very careful to spell out the semantics in the RFC.
Alas, it looks like people are aren't following the rules laid down in the RFC,
which makes makes this mechanism an accident waiting to happen, just like
the ad-hoc things that preceeded it.