When a custom Polkit rule is set to disable sleep and hibernation the buttons for both sleep and hibernate are removed from the Application Launcher's "Leave" menu. However, upon selecting an option from the "Leave" menu the splash screen that appears still has a "Suspend" button (although it doesn't function). I would recommend that the splash screen respect the same Polkit rule the Application Launcher does for consistency.

System Information:
OS: Arch Linux x86_64
KDE Plasma Version: 5.9.2
KDE Frameworks Version: 5.31.0
Qt Version: 5.8.0

Polkit rule contents (/etc/polkit-1/rules.d/10-disable-suspend.rules):

polkit.addRule(function(action, subject) {
    if (action.id == "org.freedesktop.login1.suspend" ||
        action.id == "org.freedesktop.login1.suspend-multiple-sessions" ||
        action.id == "org.freedesktop.login1.hibernate" ||
        action.id == "org.freedesktop.login1.hibernate-multiple-sessions")
    {
        return polkit.Result.NO;
    }
});