src/widgets/util/qsystemtrayicon_x11.cpp is built whenever the xcb backend is enabled. It connects to QXcbNativeInterface::systemTrayWindowChanged but Wayland doesn't provide QXcbNativeInterface obviously what results in the warning. I guess it has to check what qGuiApp->platformNativeInterface() actually implements.

To reproduce the warning, one has to launch any Qt application creating QSystemTrayIcon condition-lessly (i.e. even if QSystemTrayIcon::isSystemTrayAvailable() returns false) on a Wayland system without tray.