Regen Braking Threshold before Friction brakes kick in

The example in the top figure is for heavy braking though because you wouldn't see friction braking at higher speeds otherwise. Therefore, we don't learn much from it on what happens in gentle braking.

I just tested what happens on my Gen 4 Prius Prime. You can see regenerative braking on the display down to 2–3 mph on the speedometer depending on the driving conditions. Note that the speedometer overestimates the speed—by 2 mph at higher speeds, but I am not sure by how much at lower speeds. Interestingly, the threshold is higher with the ICE on.

So, on Gen 4 Prius Prime, regenerative braking exists down to 2–3 mph if not lower. It is not clear how much of the energy goes to friction braking at such low speeds. According to reviews, brake modulation on Gen 5 is nearly perfect; so, the threshold may be even lower on Gen 5.

 

The posted graphs do not have values nor units. They are just for illustrative purposes of how the two brake systems work together. It is a blended system. Not an either or one. Regen braking doesn't stop when the friction brakes are applied. It stops when the battery can't accept a charge, or the car stops spinning the M/G. The dash display does not show when the friction brakes are applied. It showing regen down to 1mph does not exclude the friction brakes being applied at a higher speed. Polling rate is also something to consider with the display.

 

Sampling rate of the display for that screen is quite fast according to my observations yesterday, in line with that of the speedometer.

OP's question was whether he had to worry about the friction brakes being applied at as high as 18 mph during deceleration. The answer is not if you drive gently. As I mentioned, I get 115 mpg round-trip in stop-and-go freeway driving on my Gen 4 Prius Prime, and that wouldn't be possible if friction brakes were generously used in low-speed driving.