That's not legal, and it doesn't happen in cases of the kind of workers you are talking about. Taking passport or treating to be accused of rape and similar things happen (not commonly) to domestic workers and drivers. In occasions they buy the right of Kafala from the Kafil for a couple of $1000s and you leaving before your contract ends feel to them like a loss of money, I would say the situation here is indeed similar to slavery. For industry workers, every worker is replaceable and the company hiring you has 0 incentive to force you to stay.

The real issue in gulf is not paying livable wages mainly. Anything else is comparable to farm workers in Europe.