Hi,
this is a duplicate of #798520 which was already solved, but as you
said
> This file was however not properly cleaned up.
is correct. I see an error on upgrading from jessie to stretch:
Preparing to unpack .../archives/debtags_2.1.2_all.deb ...
dpkg-query: package 'debtags' is not installed
Use dpkg --info (= dpkg-deb --info) to examine archive files,
and dpkg --contents (= dpkg-deb --contents) to list their contents.
Unpacking debtags (2.1.2) over (1.12.3) ...
and this seems to block the dpkg-maintscript-helper when it should
remove the old config files.
This is just a quick check to confirm the bug.
Thanks for reporting.
E.
--
GPG key: 4096R/5E0195FAF2133176 2010-10-19 Enrico Rossi