The debate on having this shortcut is long on going so I’m not going to get into that. But I use it and it saves me loads of time. When it gets disabled it is a right pain. Personally I like the warning box to pop up too which is not possible to restore unless you edit the registry.
It took some diff’ing of the registry but the key you want to undo either clicking never show the box again, or changing the settings in the Advanced E-Mail Options, is under
Delete the CtrlEnterSends key and it’ll bring back the warning box when you restart outlook.