The tag is present in Lintian version 2.104.234. That is the most recent version we know about.
This package contains a
postrm maintainer script that uses
an interpreter that isn't essential. The
purge action of
postrm can only rely on essential packages, which means the
interpreter used by
postrm must be one of the essential ones
Refer to Debian Policy Manual section 7.2 for details.