Created attachment 104825 [details] Testprogram Attached small Qt application shows that kwin_x11 always shows a system menu and a minimize button although I only set the flags to show only the title: Qt::CustomizeWindowHint | Qt::WindowTitleHint; xfwm4 and icewm also show the system menu (which I think is also wrong) but at least both of them do not show the minimize button but kwin_x11 does. Note that kwin is used here in an industrial application where more stringent rules shall apply what the end user shall be able to do with the windows.
KWin does not have support for these aspects of the non standard and not documented hints of the Motif Window Manager. This is a custom protocol between Motif applications and the Motif Window Manager. KWin never had support for this and will never have support for this. The hints are not suitable for KWin as KWin does not have any control over the window decoration. The decoration is provided by a plugin, thus outside the scope of KWin.