I see no Return-Path headers in some IETF traffic, including
<87znsvwlxa(_dot_)fsf(_at_)snark(_dot_)piermont(_dot_)com> as it appears in
my mailbox.
Instead I see an old UNIX-style From_.
That's because Return-Path: is frequently added at final delivery. My
MTA adds that, some other don't. (Many Unix MTAs use the From_ line).
Note that the ones that don't add return-path are in violation of
over 20+ years of mail specificatons. See RFC 821 (top of page 22),
RFC 1123 (section 5.3.3 among others), and RFC 2821 (section 4.4).
However there's no guarantee that Return-Path is either consistent
for all messages sent to a list, nor that it is uniquely associated
with a list. The few standards that apply to lists only require that
the return-path address serves the purpose of informing the list
maintainer (perhaps a robot) of nondelivery reports.
Keith