1. slavery became illegal, except as punishment for a crime
2. a ton of vague laws sprung up, like "malicious mischief". Look up "Jim Crow" or "black codes" to get a sense of these.
3. States started "convict-leasing" out prisoners as a source of income, often right back to the plantations that slaves were liberated from before. The convicted were not paid for this labor.
Additional context: Virginia Supreme Court rules that inmates are slaves to the state in 1871: https://www.ojp.gov/ncjrs/virtual-library/abstracts/slaves-s... Virginia held the capitol of the Confederacy - the states that tried to leave the USA to retain their slaves.
I forget why the crime exception was added to the 13th amendment, but I assume it was to make it more palatable to the states that still wanted slaves
