Default list signature blocks do not follow good netiquette

Mailman list footers are not intended to be signatures.

Because they're not signatures they are included by MUAs in every reply. That reply typically goes back to the list and another list footer is added, that also then gets quoted. See http://lists.edlug.org.uk/pipermail/edlug/2017-February/000431.html

The list footer should be intended to be a signature. If the user has a signature then that will precede it and just increase the one signature's size and it will all be stripped in the reply, which is desirable. Many newcomers to mailing lists aren't aware of the signature convention so Mailman should default to using one and explain what it is and why.

I understand the issue and I could easily implement this, but I won't. This is only one part of a larger problem which is that top posting with full quoting makes a mess on email discussion lists.

The main reason I won't change this behavior is there are likely existing lists that don't want the information in the footer to be treated as a sig. With current behavior, the list owner has the ability to make the footer a sig if desired, and to not make it a sig if not desired.

This would still be the case if I implemented this by just adding the "-- " line to DEFAULT_MSG_FOOTER in Defaults.py, but there are multiple reasons why I wouldn't do it that way. I would do it in the code that adds the footer and only when adding the footer to the message body and not as a separate MIME part and only when the body does not already contain a "-- " line, but this would preclude the list owner making the footer not a sig.

I realise I might not have been clear what with joining onto someone
else's bug report. I agree it should be the list owner's choice and
that they already have this choice just by starting the footer with
"--␣\n", as I do.

Given that, my suggestion is DEFAULT_MSG_FOOTER in Defaults.py gains
that prefix so new lists start with a sig and the owner can change it to
non-sig rather than the other way around. Existing lists certainly
shouldn't be switched. Many list owners don't know about sigs so it's
not as if they've made a conscious decision to stick with Mailman's
non-sig choice. Making sig the default for new lists would help these
owners get it "right".

I take your point about not if in a separate MIME part, etc., but my
manual sig setting makes it through to a separate MIME part today so
altering DEFAULT_MSG_FOOTER would be no worse off. Implementing a new
"make footer a sig" Boolean, with all the considerations you mention
taken into account, sounds more 3.0-ish. (I've no idea what 3.0 has
done in this area.)

So prefixing DEFAULT_MSG_FOOTER so an owner can delete the sig prefix
seems an easy change and gives a result no worse than my persuading each
owner individually that they should switch, once they learn the sig
technology exists.

A weaker alternative would be to alter DEFAULT_MSG_FOOTER's help text to
explain how to make it a sig, and why it's beneficial, referencing
https://en.wikipedia.org/wiki/Signature_block#Signatures_in_Usenet_postings

The values for DEFAULT_DIGEST_FOOTER and DEFAULT_MSG_FOOTER have been changed to use a standard signature separator for DEFAULT_MSG_FOOTER and to remove the unneded line of underscores from DEFAULT_DIGEST_FOOTER.