Involuntary labor (slavery by another name) is still a valid form of punishment in many jurisdictions. Some prisoners are forced to work whether they want to or not, and they're not paid for that work. Four of the states in question removed that option from their constitutions. https://www.pbs.org/newshour/politics/voters-in-4-states-reject-slavery-involuntary-servitude-as-punishment-for-crime The fifth state actually rejected their amendment due to language issues, but are planning on bringing it up again in the future.

So no, it's not just a feel good vote, it does have practical implications for prisoners. As the PBS article I linked mentions it's a bit more nuanced than, "This immediately ends all forced prison labor", but it lays the groundwork for challenges.