3-phase motors are controlled by torque commands into the driver. Give it a value and it generates requisite voltages to fill the gap between current state and desired state. Give it a positive value and the driver spins up the motor, give it a negative and it artificially spins down the motor progressively by commanded amounts. So especially off-throttle regen is completely free. IIUC.

It's the same control system that operates the motor. The motor is just being used as a generator.

I'm not sure which pressure sensor you mean, like in the brake lever? E-bikes with hydraulic brakes already have sensors for power cutoff (and in this case for brake lights).

If you engineer it properly, it doesn't add _any_ weight or complexity. All you need is a bit different arrangement of power transistors and some software.

Why existing bikes don't use it? Because you need software or a more complicated controller, and the amount of regenerated energy is indeed not that large.

Are there popular e-bikes without electronic motor control?

It was already going to have some sensor for, say, the brake light.