The tag is present in Lintian version 2.104.318. That is the most recent version we know about.
This .dsc file contains no Checksum-Sha256 field and hence only weak digests.
This issue will only show up for source packages built with dpkg-source before 1.14.17 (March 2008) and hence will probably never show up when you run Lintian locally but only on https://lintian.debian.org/ for source packages in the archive.
Accordingly it can be fixed by simply rebuilding the source package with a more recent dpkg-source version, i.e. by uploading a new Debian release of the package.