Open AI uses mechanical Turk, which is the closest you can get to slave labor while still technically paying somebody, except for prison labor I guess.
Yup...I've had the displeasure of being on that thing for a few months some years back. I couldn't shrug the feeling of feeling like a sweatshop worker. "Ah yes, do these menial tasks that'll take you 40 minutes of your time to earn...5 cents! woo!"
They originally paid Nigerians a pittance to look at horrific material all day to weed it out of the training and gave them very little in terms of mental health treatment for them.