The tag is present in Lintian version 2.104.234. That is the most recent version we know about.
The directory /usr/lib/site-python has been deprecated as a location for installing Python modules and may be dropped from Python's module search path in a future version. Most likely this module is a private module and should be packaged in a directory outside of Python's default search path.
Refer to Debian Python Policy section 2.5 for details.