IANAL, but I'm not sure that's exactly true. As far as I know, in 49 states (not Louisiana), if there's not a law about it we fall back on some system based on english common law.
In general, each sovereign jurisdiction will fall back on the legal system that was in place at the time the jurisdiction gained its sovereignty. For California, the ancestor state was Mexico, and thus Spain.
https://www.law.berkeley.edu/library/robbins/pdf/ca-legalher...
But to answer the larger question. Everything that is not specifically legislated against is legal. Some ancient laws may still hold, but most are probably going to be found unconstitutional. Although, there may be some exceptions that you could convince a court to enforce.