If we're talking forced slavery, I'm against it. But if we're talking a contract, then I don't think there's anything wrong with it. Many sub cultures in fetish have such things. From domination and slavery straight out, to the subtle slavery where you are a servant, to blood contracts where you give someone blood and until they're done with it they're yours - or for longer.
Break the contract, and they're released. Some even go threw long talks about it and literally set it up as a legally binding contract - though I don't think it would be allowed, still the point is clear. Some reputations are handled, based solely on these kinds of relationships, and not how "mean" the master was or "evil", but rather how they and the other party decided to split. Someone's entire reputation in an area could be ruined. Some clubs will not allow them in anymore.
Not a big problem, it would seem, but when your entire fantasy revolves around a place/person/fetish that is so commonly associated with evil, you're going to be pretty damn upset when one of the not-so-common places decides to throw you out for good. Others catch on, and might follow suite. You're left alone.
But, unless you get the extreme - which is much more rare now-a-days and wasn't really as common as the media darlings would speak of, you're not going to get a case of "you're my slave, and you'll never get out!" and in a meant manner. Part of the domination is language, you may hear people say that - but it's for fantasy.
Gangs have it, thought it was more common a decade or two ago where if you shamed so bad but wanted to bargain for your life, you'd become a slave - which is completely different. This is slavery in our most common thought. Work with no pay, beatings, everything - and for most there was no way to run. It was this or death.
Some, a famous case where a guy nicked Purp was strung up and they'd come and shoot his veins up with drugs to check the quality of high. Disagree with that, I do, this is forced slavery - even though Purp decided to, he still had no choice: do or die. But anything between two willing people - key word willingly, that is adults who are fine with doing so - is there business.