This feature has been broken since:

commit a2c24879e19dad1d261c42bcd16c0960b330b6b3
Author: Luboš Luňák <l.lunak@kde.org>
Date:   Mon Apr 2 13:02:51 2007 +0000

    Have fun supporting setSystemTrayWindowFor() with XDG systray.

Apparently nobody is missing it if it has been reported only once over
the last five years.

CCBUG: 180062
REVIEW: 109219