> You can't buy and sell them. You don't have them indefinitely. When they reproduce your don't own their offspring. They get sentenced to this servitude by society's justice system, not captured through conquest or purchased from foreign lands.

All of these individually are features of historical systems uncontroversially identified as slavery. The particular form of penal slavery at issue may be unique in combining all of them, but I don't see any coherent argument why that would make it into not-slavery.