The tag is present in Lintian version 2.104.244. That is the most recent version we know about.
This package build-depends on a patch system such as dpatch or
quilt, but there is no
debian/README.source file. This file is
recommended for any package where
dpkg-source -x does not result
in the preferred form for making modifications to the package.
If you are using quilt and the package needs no other special handling
instructions, you may want to add a
/usr/share/doc/quilt/README.source. Similarly, you
can refer to
/usr/share/doc/dpatch/README.source.gz for dpatch.
Refer to Debian Policy Manual section 4.14 for details.