It is less than the 11.2 cutoff given above for the battery pack, and it is more than the point where we will get random attempts to reboot the camera when there is too little power to succeed. It is also a good figure for if someone wants to use an unprotected lithium 4S battery.
The last time I tested a Viofo converter, it did run on 9V, failed at about 8.5V, which meant that I could run the dashcam on the 9V output of a USB-C PD powerbank, which was probably a little more efficient than running it at 18V. That powerbank didn't give a 12V output option.
So I can see that it can be useful to have a complete disable, for when I want to run a Viofo dashcam that doesn't accept 5V USB and I am using a USB powerbank that refuses to provide 12V, but I have only met one Viofo dashcam that doesn't accept 5V.
(The MT1 if you were wondering, I still have that on my bicycle powered at 9V from a USB-C PD powerbank.)
The question is, what is the most useful cutoff voltage?
And is that voltage more useful than 0V? 0V seems completely useless to me!