In order to not disrupt long term users, and to create further momentum for switching to wayland, 
I kindly ask for implementation of the window shaded feature similar to what we have in X11.

X11 does manage a boolean (toggle) flag called SHADED. If active, the window is drawn with the window content height set to zero. The window geometry isn't affected from this flag, and as such, such a window could be persisted in the session handling. After session restart, such a window is restored with its original size, but again, the screen representation is restored with height zero. 

The popularity of https://bugs.kde.org/show_bug.cgi?id=450582 demonstrates, that a lot of long term users rely on this feature and again, such an implementation would be required to allow them to switch over to wayland without friction.