> I used an e-bike (without a licence plate) last week in Tokyo.

Did you (1) need to pedal to get assistance, or (2) could you get power with a throttle button only? If #2, then you were breaking the law. It seems like police are not yet enforcing. In neighborhoods with a lot of "night life", I see this often with host-looking dudes. I expect 6-12 months after the law is activated, police will begin to crack down. (This is a pretty normal pattern when introducing new traffics laws in Japan.)