For the question: are they slaves? To some degree they allowed themselves into that. They knew going in they couldn't "opt" out. Some like Alpha who were prison inmates recruited I could see as more slaves than those who agreed, either willingly or under duress.
To me this question is what makes shows like this good. Some moral ambiguity to make us all think and rethink what we believe we already know as definitions for things.
My main answer: NO
I voted yes mainly because actives have no way to "opt out" of their contract. And like someone said above, the company can simply keep actives around long past their contracted period of servitude quite simply because the actives would have no recollection of the contract, or its duration.
I don't recall off hand now but did Millie/November seem to act as if she knew Adelle and had fulfilled her contract at the season close? I know I don't recall her, at the point we come into the conversation, acting weirded out by what was going on.
I have a thought on that scene: Spoiler