Forced labor and slavery aren't really different in the context of the state owning people. That's why it's phrased like that in the constitution. Once you are found guilty of a crime in the US, you lose many of your rights and are considered a prisoner of the state or federal government.

It's actually a hot topic in the US anyway, because the government very often assigns its prisoners it privately owned, for-profit prisons, where those prisoners labor for pennies and have no choice. Here's an example for you.

Is it involuntary servitude or slavery when the state hands you off to a private prison to make them money?

Couple that with the fact that the US coincidentally has the highest incarceration rate in the world (not crime, just the act of putting people in prison) and the fact that private prisons very often sign contracts with the states for a minimum number of prisoners a year, and you can see that it might be argued that private prisons collaborate with the government as an institutional system to keep certain Americans in prisons.

And then there's the fact that poor and non-white people are disproportionately preyed on by police, maybe you could say that modern day slavery still exists.

Involuntary servitude might be morally ok, but there's still a line where it crosses into slavery and we've been on the slavery side for a long time now.

Indeed, according to this article (emphasis mine):

Incarcerated workers generate billions of dollars worth of goods and services annually but are paid pennies per hour without proper training or opportunity to build skills for careers after release, according to a comprehensive nationwide report released by the University of Chicago Law School’s Global Human Rights Clinic and the American Civil Liberties Union.

The first-of-its-kind report, Captive Labor: Exploitation of Incarcerated Workers, examines the use of prison labor throughout the U.S. and highlights how incarcerated workers’s labor helps maintain prisons and provides vital public services.

[...]

Key findings include:

• Nearly two-thirds (65% percent) of incarcerated people report working behind bars—amounting to roughly 800,000 workers incarcerated in prisons.
• More than three quarters of incarcerated people surveyed (76%) report facing punishment—such as solitary confinement, denial of sentence reductions, or loss of family visitation—if they decline to work.
• Prison laborers are at the mercy of their employers. They have no control over their work assignments, are excluded from minimum wage and overtime protections, are unable to unionize, do not receive adequate training and equipment, and are denied workplace safety guarantees despite often dangerous working conditions.
• As a result, 64% of incarcerated workers surveyed report worrying about their safety while working; 70% percent say they received no formal job training; and 70% percent report not being able to afford basic necessities like soap and phone calls with prison labor wages.
• Incarcerated workers produce at least $2 billion in goods and $9 billion worth of prison maintenance services annually, but this number is not closely tracked and is likely much higher.
• Yet, most states pay incarcerated workers pennies per hour for their work. Seven states (Alabama, Arkansas, Florida, Georgia, Mississippi, South Carolina, and Texas) pay nothing for the vast majority of prison work. Other states pay on average between 15 and 52 cents per hour for non-industry jobs. Prison laborers often see up to 80% of their paycheck withheld for taxes, “room and board” expenses, and court costs.
• More than 80% percent of prison laborers do general prison maintenance, which subsidizes the cost of our bloated prison system. Other tasks represent less than 10% percent of work assignments, including: public works projects (like road repair, natural disaster assistance, forestry work, and maintenance of parks, schools, and government offices); state prison industries, agricultural work, and coveted private company work assignments.

It is also worth noting that:

"Prison in-sourcing" has grown in popularity as an alternative to outsourcing work to countries with lower labor costs. A wide variety of companies such as Whole Foods, McDonald's, Target, IBM, Texas Instruments, Boeing, Nordstrom, Intel, Wal-Mart, Victoria's Secret, Aramark, AT&T, BP, Starbucks, Microsoft, Nike, Honda, Macy's and Sprint and many more actively participated in prison in-sourcing throughout the 1990s and 2000s.

And that black people are five (5) times as likely to end up in jail.

John Olivier and his staff did a great piece on it.

But isn't slavery the use of people to preform labor? I don't think they do that anymore.