The tag is present in Lintian version 2.104.328. That is the most recent version we know about.
The key on this line of the desktop entry is not one of the standard
keys defined in the FreeDesktop specification, not one of the legacy KDE
keywords, and one that does not begin with
X-. It's most likely
that the key was misspelled.
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/desktop-entry-spec/latest/ar01s06.html for details.