Don’t pass laws you won’t be able to enforce. Cat people will never observe this law. You know it, and I know it.
71%-94% of the population must be sterilized for it to be effective, assuming humans don’t add to the number of feral cats. Many of such programs have been unsuccessful according to my cite above.
Partially effective programs can still be worth supporting. Also, the best way the gauge their level of effectiveness is to give the law a try. Afterwards, more traditional measures can be attempted.