Depends on the sort of slavery we're talking about, I guess.
You've got indentured servitude slavery (people selling themselves to pay off a debt or to work), military slavery (taking captives after battles and selling them to support the war effort), and kidnapping slavery (grabbing free people and selling them as slaves). That last one was typically seen as being bad. Kidnapping was frowned on or outlawed in most ancient civilizations.
Most of the population of the Roman Empire were slaves, but they tended to get some degree of compensation that they could save up and use to buy their freedom to become a Roman citizen. That's one model if you want slavery to be less awful.
Then you have serfs, people who don't own their land but work it. While 'technically' free, they may as well be owned by their feudal lord. We already know Ionia has slavery of this form.
Or the sort of cast-like slavery you had in the Americas, where you were a slave along with all of your offspring the end. That seems to be the sort of slavery found in ancient Shurima, where slaves didn't seem to have any rights at all.
And if you want to be SUPER awful, you can go with Arab-style slavery where the men were castrated and the women were very not castrated. Thankfully Runeterra doesn't have this form of slavery going on to my knowledge.
Then of course there are various systems of 'totally not slavery' like people in company housing in debt to company stores. I can see something like that happening in Zaun/Piltover.