Not sure, but you might still have power steering if you leave it on but in neutral. If you turn it off, you'll definitely lose hydraulic steering assistance.
I have dealt with 1 modern-ish/heavy car that stalled its engine on a downhill slope and my braking ability in this case was reduced to a mere suggestion. If you are strong enough to bring down an unpowered 4000lb+ vehicle to a complete stop on a downslope, more power to you but it is a bit unfeeling to assume that everyone shares your extraordinary pedal-pounding strength. Edit:s/freakish/extraordinary.
You have a tiny master cylinder compared to a vehicle with power brakes, so it should work fine... this is different than having a big master cylinder + power brakes disabled.
At any respectable speed in a well constructed car with a mechanical steering linkage, power steering is basically unnecessary. My old car used to have glitchy power steering— at 35mph, I could generally tell when it wasn’t working, but it wasn’t very obvious. At 5 mph it was quite obvious when it wasn’t working.
It’s really dependent on the car. I was driving a C7 Z06 and the power steering cut (it overheated—car warned me well ahead of time). Let me assure you it was not trivial at all to steer the thing. It was shocking how heavy the steering became. I could turn the wheel—barely. Would not really call it control.
The next day my friends e90 BMW m3 power steering boiled over. And another friends F80 M4 had multiple power steering failures. They weren’t exactly happy about it either.