The replies to this are kind of surprising, in that apparently many USAmericans don’t know their own constitution. To be clear, this isn’t just a California thing, the Thirteenth Amendment to the US Constitution, which ‘abolished slavery’, reads:

Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.

Slavery is explicitly still permitted in the US, it has just been restricted in scope.

In the US, black people are incarcerated at five times the rate of white people. In the US, there are 2.3 million black people under ‘correctional supervision’ - this is more than 5% of black people in the US.

74% of people in US jails have not been convicted of any crime.