The tag is present in Lintian version 2.109.22. That is the most recent version we know about.
This package Build-Depends on the Python 2.x version of the Sphinx documentation generator.
The 2.x series of Python is due for deprecation and will not be maintained by upstream past 2020 and will likely be dropped after the release of Debian "buster".
Some Python modules may need to depend on both
python3-sphinx but please consider moving to only Build-Depending on
python3-sphinx package instead.