The tag is present in Lintian version 2.109.24. That is the most recent version we know about.
The package installs a __pycache__ directory, which is normally only used to store compiled Python source files. Compiled Python source files must not be included in the package, instead they should be generated at installation time in the postinst.
Note this tag is issues even if the directory is empty.
Please refer to Debian Python Policy section 3.7 for details.