Legacy admission students aren't usually that quiet about it. In my college experience, a lot of them will tell you of their legacy stars with zero prompting. I don't think that penalty will inhibit universities at all.
Instead, start cracking down on collegiate sports for colleges that break the law. UCs and CSUs will comply with this if you write the law so that college athletes are prohibited from playing ranked sports with other colleges until all legacy admissions are banned and anyone who got in via legacy is kicked out. But of course Newsom would never do that - it's catastrophically unpopular! He can't stand being the tiniest bit unpopular, he's gonna try to be President someday! Screw that guy.