Here's how to reproduce it using only packages from the archive:
$ lintian --no-cfg -I debconf_1.5.59_all.deb | grep -w unused
I: debconf: unused-override postrm-does-not-purge-debconf
$ lintian --no-cfg -I debconf_1.5.59_all.deb debconf-doc_1.5.59_all.deb | grep
-w unused
[nothing]
But it turns out the latter is non-deterministic. About half the time I
get:
$ lintian --no-cfg -I debconf_1.5.59_all.deb debconf-doc_1.5.59_all.deb | grep
-w unused
I: debconf: unused-override postrm-does-not-purge-debconf
--
Jakub Wilk
