To be clear, when the OP wrote "Japan has bicycles with license plates", it is important to clarify the term "bicycle". It would more accurate to say "motorized bicycle". If you ride something that looks like a bicycle where you can power it only with a throttle button (no pedalling required), then it requires a license plate, at least in Tokyo. Explanation here: https://www.city.inagi.tokyo.jp/en/kurashi/zeikin/1002693/10...
Also, you can ask Google AI for more sources and info using this prompt:
> 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.)
Also, you can ask Google AI for more sources and info using this prompt: