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.