I know all about must vs should etc, but clearly 1153 envisioned trimming
headers to the form you actually see them in text digests (it was, after
all, a de facto RFC rather than a speculative one).
I stand by my earlier comments - the text Digest as a readable daily
omnibus of a list's traffic is a paradigm that has stood well for over a
decade, and will continue. Bursting, MIME "digests" etc, are all
speculative bits of cleverness that programmers thought up in an attempt to
"improve" on something that was already working and that people liked.