In one case, my parent's car's ECU (we drive the same make/model) blown one of its coil signalization lines. In that case the car has no backup for example. You need to tow it, because it can't understand that it's output is gone, and can't get into proper limp mode. You need to tow the car.
Cylinder disable on overheat and other modes are proper "limp modes" which keeps car operational when other systems get damaged, but not the ECU.
The automatic transmission has its own limp mode too, which is activated by selecting the 3rd gear and making some other internal pressure adjustments.
"2001 Ford Focus MK.I"
Looks like later switched to Bosh https://www.ecufile.net/ecu-file-description/BOSCH-EDC16C34-... https://www.ecufile.net/ecu-file-description/BOSCH-ME9.0-026...
no second ecu hidden inside
I had to use at least one of these modes twice during the ownership of my car, too. My ignition coil's one output went dead, twice. Once overnight, once during driving at highway speeds. In both cases I went to where I need to go safely, because the car knew what to do, and the facilities to do so.
These features are written in the user manual as fine prints, but they're not sales points. Nobody advertised these to me.
Tesla is selling an autonomous, connected car with enough electronics to fly a small plane, and they rightfully add redundancies inside it. That's great, that's the way it should be. Then they're removing a redundancy feature which might save lives even if it's rarely used.
It's akin to MCAS system. "It doesn't change the behavior, so the pilots don't need to know". We know how well it went.