The KDE Daemon global shortcuts allow to choose a shortcut for Take Rectangular Region Screenshot, it defaults to Meta+Shift+PrtScr.
However that takes a screenshot with the mouse pointer, how do I disable that?
When I open Spectacle, the includeMousePointer is not checked, so apparently that's a different setting.
Apparently this commit seems to say it's possible to do so but I don't know how.
If you press the hotkey, a D-Bus command is invoked. These commands accept arguments instead of looking at configuration files for maximum flexibility.
The actual D-Bus command that you would need to invoke is:
qdbus org.kde.Spectacle / org.kde.Spectacle.RectangularRegion false
The argument for this RectangularRegion method indicates the value for includeMousePointer. The commit you referenced indeed makes the default hotkey action call this with a true argument instead of false.
If you want to change the default, edit spectacle.khotkeys file in your system. Locate the [Data_1_4Actions0] section, and change Arguments=true to Arguments=false. The result should look like this:
[Data_1_4Actions0]
Arguments=false
Call=RectangularRegion
RemoteApp=org.kde.Spectacle
RemoteObj=/
Type=DBUS
It might be required to restart your session after changing this.