The tag is present in Lintian version 2.109.3. That is the most recent version we know about.
A paragraph in debian/tests/control mentions an obsolete value for the Restrictions field. Though still allowed, this will become unsupported in the future and the whole paragraph will be ignored.
Please refer to https://salsa.debian.org/ci-team/autopkgtest/tree/master/doc/README.package-tests.rst for details.