The tag is present in Lintian version 2.109.24. That is the most recent version we know about.
The package uses a patch system, but the Debian diff.gz contains
changes to files or creation of additional files outside of the
debian directory. This often indicates accidental changes that
weren't meant to be in the package or changes that were supposed to be
separated out into a patch. The package will also more easily support
possible future source package formats if all changes outside the
debian directory are stored as patches.