@EL
Maybe slavery isn't the right word but like you said a lot of the time it isn't working by choice, and I don't mean in the 'I hate this job but I need to pay the bills' way, in a 'I'm being coerced and they use violence to stop me leaving' way
Let's call that "heavily coerced" - one small step from slavery. Yes, there are jobs (sexual and non-sexual) where this happens.
I've already decided that if I get to build a house, I'm personally visiting the place that makes the bricks to be sure that no one there is subject to any coercion above the need for an unpleasant job to pay their bills. If I ever encounter even a hint that a massage place in my area might be doing this, I'll try to investigate and, if the problem was confirmed, will try my best two ruin the lives of the people behind it.
I'm sure there are some abusive massage places, but through friends translating and the few foot massage girls I've known who speak English, I'd say it's far from an overwhelming issue, at least in my little corner of the world. A standard foot massage contract in my area is:
Monthly base salary of 2500-4000 (depends on the place, plus data acquired over time) for working a LOT of hours. This is equal or better than a lot of factory office jobs.
Work above those is rewarded at a higher per-hour rate.
Work below those hours is penalized at more than the per-hour rate.
Number of days allowed off without some sort of penalty is 2 or 3 per month (better than a certain high-end dental office I know of).
There's a dorm provided. Yes, somewhere there are dorms full of foot massage girls.
Of course, if some naughty client bribes a girl for a happy ending, I don't see how the bosses are going to find out how much she collects unless they thoroughly search the girls every time they leave the place.