Request: revive nnimap-split-predicate or variation thereof

[Matt Lundin <mdl@imapmail.org>]

I've been playing with the new nnimap split setup and find that
splitting is hard-coded to work only on unread messages (see the
function nnimap-new-articles).

A problem arises when I read mail via webmail. Currently, anything
marked as seen in a webmail session will not be split by Gnus, thus
remaining in the INBOX in perpetuity unless it is manually refiled or
marked as unseen.

With older Gnus, it was possible to specify which messages one wanted
split with nnimap-split-predicate (e.g., UNSEEN, UNDELETED). I find the
following justification for the variable in the old manual:

,----
| This might be useful if you use another IMAP client to read mail in your
| inbox but would like Gnus to split all articles in the inbox regardless
| of readedness. Then you might change this to ‘UNDELETED’.
`----

Might I request reviving a variable that allows users to customize which
messages are considered for splitting?

Thanks,
Matt