I was talking about a possible custom function that allows ISO to drop from a manual setting to avoid over-exposure. Canon started including this a few years ago.
It is not available in M mode in the cameras I have that have it.
Just checked both 6D and 80D. In Manual mode, ISO set to 400 and Safety-Shift set to ISO, both cameras will allow extreme overexposure, well past blown highlights. ISO setting remains at 400, even for the overexposure.