No possible way to disable shadows without disabling compositor
The checkboxes under "window manager tweaks" settings, "compositor" tab. Only the shadows under docks (panels) can be disabled. Toggling the other checkboxes for shadows under popups (menus) and regular windows - or editing the corresponding settigns using xfce4-settings-editor -> xfwm4 - doesn't disable them, even after dis/re-enabling the compositor, or even logout/reboot.
X11, same result on several different machines with different graphics cards, mainly nvidia, intel-builtin.