I doubt there's a limp mode home, that would probably be an EPA violation
Just for comparison, our newest trucks at work also have the dreaded DEF of course, but at least a gauge, anyway, they have now put a pressure switch in the cooling system that will completely shut down the truck if it starts to lose coolant. In my experience, that's what a temperature gauge and a warning light are for, but I guess the fools building vehicles these days don't consider the fact that you might need to get out of the roadway? I mean the truck will simply shut down. Seems absolutely stupid to me to have these 'safe' minders built in to our vehicles.
Another thanks to our lawsuit happy brethren here I guess.