The tag is present in Lintian version 2.104.319. That is the most recent version we know about.
This desktop entry uses OnlyShowIn to limit the environments in which it's displayed but lists multiple environments. This is often a sign of a desktop file written assuming that only GNOME, KDE, and Xfce are in use and the desktop file intended to exclude one of them. This unintentionally hides the application from desktop environments such as LXDE where it would work fine. If this application supports any desktop environment except some specific ones, it should list the unsupported environments in the NotShowIn key instead.