> debt has been repaid

I know people say this, but I think this framing likely generates anti-prison arguments because it basically doesn’t make any sense. How does being in a cage for X years repay society? It doesn’t. It does keep the harmful person away from society though, which is a very different and useful thing (in many cases, obviously imprisonment for some crimes is dumb).

Being in prison is the punishment. It is not restitution, but as part of the punishment restitution could be imposed. It's hard to pay that restitution while incarcerated though. Some people advocate that just because one has been released from incarceration that they should still not be allowed to vote until any moneys owed have been paid. That could be fines from the court as well as restitution to victims.