You're going to have a difficult time pulling a lot of energy out of the back wheel as you're slowing down. The more you decelerate the less weight you've got on your back wheel. Eventually you reach the maximum energy transfer from back wheel contact patch into the motor and lock up the back wheel, and even then you may not have considerably slowed the bike.

Regen on the front wheel would be most effective - but then you've got two motors or a less-than-ideal front motor that adds unsprung weight and has similar traction issues during acceleration as the front unloads.

It's a shame - I think a lot of people want ebikes to work, but they're not as convenient as a pedal bike (especially not in small apartments) and usually they're too heavy to really use in blended pedaling/e-assist mode.

ABS on ebikes exist already, so I'm not buying your locking issue. Also e-scooters like Lime had regen braking forever.

It isn't free. How could it be free? It requires at least an electronic control system and a pressure sensor.

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.

As other commenters noted, rear wheel regenerative braking doesn’t work very well. But there are more problems: most mid-drive e-bikes fundamentally can’t regeneratively brake at all: the rear hub freewheels and cannot drive the motor. Even ignoring that, the chain/belt frequently also can’t drive the motor because that would cause the pedals to drive the motor, and a lot of e-bikes are designed to be pleasant to ride with the motor off, and the rotor has rotational inertia and often has drag as well.

It's not free in a bicycle - it requires significant design compromises in the drivetrain because normally the rear hub has a freewheel to keep it from being back-driven (this may be part of the reason they went for "pedal-by-wire").

On a vehicle like this, which resembles a bicycle but is not one, it's free to implement regenerative braking. It already has all of the necessary components.