As of 2023 municipalities and counties can no longer mandate bicycle registration. (See https://law.justia.com/codes/california/code-veh/division-16... as amended by sec. 7 at https://leginfo.legislature.ca.gov/faces/billTextClient.xhtm...) Though universities, like UC Davis, might still be able to require it for bikes on campus.

I hadn't heard of the requirement before. Mandatory registration originally seems to have been intended to address bike theft. All bicycles sold in California must have a serial number. A significant number of cities (most?) had ordinances requiring registration. But few people knew about it and even fewer registered their bikes.