The tag is present in Lintian version 2.104.328. That is the most recent version we know about.
This desktop entry uses a type that's not one of the currently recognized values of "Application", "Link" or "Directory". Implementations should ignore any unknown values but it's still likely an error if you used something else and this check should be updated if any new desktop file types start being used. Note that case is significant.
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.