In theory, the Emancipation Proclamation of January 1, 1863, along with the Union defeating the Southern rebels, and in addition to the 13th-15th Constitutional Amendments, secured forever, freedom and full citizenship for those previously enslaved. Of course, as always in America, there is the law as written, and there is the reality of the law, as practiced. Still, credit must be given where credit is due, and in significant ways, it can be said, that slavery, at least in the form of chattel slavery, does not presently exist in the United States.
However, the fact of the matter is that the power structure of America is a very small subset of the population, of which this small elite of people, are forever afraid that their power and their control could somehow be wrested from them, and most especially are cognizant that since America has such a high percentage of those that own absolutely nothing, with little or no opportunity to become something, that they therefore recognize that those people could at a some point, create real and systemic domestic insurrection, which could thereby possibly upend the power structure of the elite.
This means, in consequence, that the elite have merely borrowed from the past, of what worked so well for the South, of which, had it not been for the hubris of the South, in deliberately provoking war with the North, slavery might even be the law of this land, today. For the South successfully demonstrated, that a small percentage of plantation owners, few in number, could control a massive amount of slaves, as well as ruling the power structure of their communities, by being the force behind the policing factions, the justice department, and by the copious use of violence by their adherents profitably kept their slaves in their place.
Fast forward, to today's world, and recognize, that the people that are incarcerated and convicted of felonies, are most often, those that are the most indigent, ill-educated, and oppressed; in which such convictions are almost always accomplished without the alleged perpetrator even having their day in court, for seldom are felons actually convicted by a jury of their peers, but rather conviction after conviction is accomplished by "plea bargaining" in which the person so making the plea, is completely out of their element, and susceptible to every bit of pressure to do so, and have not the tools to effectively do any other.
Once a given person is convicted as a felon, irrespective of how long they have been incarcerated for, they are essentially stripped forever of their first-class citizenship rights, which has been replaced instead with something that doesn't even approach even second-class status, and is most appropriately seen as being designated as something less than fully human, and rightly seen as a new form of slavery. For, felons, have all sorts of rights taken away from them, such as no longer being eligible to vote or to be on a jury, or to work except in the most menial and dead-end jobs, or be eligible for a passport, or to be eligible for student loans or for public housing or for supplemental nutritional assistance or for various other State and Governmental programs and benefits. While it may be true, that some of these things are still available for felons, such are often amended or reduced to such an extent, that a given felon has little or no hope of ever being a fully functional, autonomous and free human being, again.
All of the above basically means that in America, while the slavery of old is no longer legal, the workaround is as simple as getting an intended target convicted as a felon, and thereupon that person becomes, for all intents and purposes, enslaved for life.