The tag is present in Lintian version 2.104.328. That is the most recent version we know about.
The specified file appears to use
py3versions -s to
determine the "supported" Python versions without specifying
python3-all (or equivalent) as a build-dependency.
With only the default version of Python installed, the autopkgtests may pass but the package subsequently faisl at runtime when another, non-default, Python version is present.
python3-all as a build-dependency.