13th Amendment
Section 1
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.
Section 2
Congress shall have power to enforce this article by appropriate legislation.
As you can see, slavery as criminal punishment is still perfectly legal.
And also notice, no specific crime, or crimes, as specified.
So, you could be sentenced to lifetime servitude, ie slavery, for any crime, from murder to jaywalking.
What do you think a life sentence at hard labour is?