The tag is present in Lintian version 2.104.234. That is the most recent version we know about.
This desktop entry lists a category that is not one of the registered Main or Additional Categories in the FreeDesktop specification. Note that case is significant and whitespace is only allowed immediately before and after the equal sign in the Category key, not elsewhere in the field.
The desktop-file-validate tool in the desktop-file-utils package is useful for checking the syntax of desktop entries.
Refer to https://specifications.freedesktop.org/menu-spec/latest/apa.html for details.