This is probably a Tomato/Tomahto situation, but you could think of her as more of an indentured servant rather than a slave.

It seems like most “slaves” in the galaxy have implants that slave-owners can detonate if they try to escape. Anakin and Shmi mention having them in TPM, and it could be another justification for why Finn and Rose couldn’t really do anything to help the slave children escape on Canto Bight for the very brief time they were around them.

Obviously Rey doesn’t have one of these implants because Unkar Plutt never retaliates. Now, I think most of the people who live at Nima Outpost are practically slaves of Unkar Plutt, since he is there only means of food on Jakku, but he doesn’t own them. So they’re more like slaves to the system rather than being property of Unkar Plutt himself. They don’t have to collect scrap for food if they don’t want to, but they’ll probably starve if they don’t.

So I’m guessing Unkar just bought Rey so she was stuck at Nima Outpost, and then had her become dependent on his junk for food system.