Since X.org’s X server, I had been unable to use the left Win key to compose keyboard shortcuts. However, while Googling around, I learnt that adding the following line to the keyboard’s
[InputDevice] section in file /etc/X11/xorg.conf will bring that support back:
Option "XkbOptions" "altwin:meta_win"
This will bring back the ability to configure shortcuts that use the left Win key, for example Win+R to invoke the “Run…” dialog box in KDE.