The tag is present in Lintian version 2.104.328. That is the most recent version we know about.
A paragraph in debian/tests/control mentions a non-standard value for the Restrictions field. Though allowed, this may indicate an error, as 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.