The tag is present in Lintian version 2.104.328. That is the most recent version we know about.
This desktop entry includes a Reserved Category, one which has a desktop-specific meaning that has not yet been standardized, but does not include an OnlyShowIn key. Desktop entries using a Reserved Category must include an OnlyShowIn key limiting the entry to those environments that support the category.
The desktop-file-validate tool in the desktop-file-utils package is useful for checking the syntax of desktop entries.
Please refer to https://specifications.freedesktop.org/menu-spec/latest/apa.html for details.