They're optional for adults in cars here in New Hampshire where the official state motto is "Live free or die".
> Records of number plates read by each LPR shall not be recorded or transmitted anywhere and shall be purged from the system within 3 minutes of their capture in such a manner that they are destroyed and are not recoverable, unless an alarm resulted in an arrest, a citation, or protective custody, or identified a vehicle that was the subject of a missing person or wanted broadcast [...]
https://law.justia.com/codes/new-hampshire/2022/title-xxi/ti...
This should be the standard IMO.
... Wait, you don't need _liability insurance_?!
https://www.nh.gov/insurance/consumers/documents/nh_auto_gui...
(Huh; I had no idea there was anywhere left in the developed world where they weren't mandatory.)