Given , Jon Doe
wrote:

There are too many factors (vehicle speed, engine load, ambient air
temperature) to determine if it's safe. The overheat risk and resultant
engine damage may not be worth it. YMMV

But that doesn't answer the question.
The question is what happens when you drive unpressurized.

All I can think of is that the boiling point is lowered.

Most engines keep the temperature around the 200 F degree mark, I think,
where even plain water wouldn't boil.

What's the boiling point of unpressurized 50:50 coolant?