It’s not a mandatory by regulations to install brake cut-off sensors. 

The sensors are for emergency incidents in cases when you brake abruptly – they’ll cut off the motor’s power. With pedal assist systems, you may not be as quick to squeeze the brake levers. In regular brakes you may need to have this as a double safety feature.  Hydraulic brakes are typically powerful enough to stop you if a panic happens.

You can try your ebike without them, and you can try your ebike without installing the cut-off and decide if you need it to be installed.