Which OP? Certainly not the one in this thread.
From the OP:
I’m wondering if there might be some tax implications. If you “gift” over any earnings you have to your master, he will have to pay gift taxes. No?
What makes you think those things can’t be valued in monetary terms?
Public policy holds, and rightfully so, that certain types of contracts are invalid on their face even if both parties willingly agree to the terms. One such contract is an absolution of all liability. For example, if you go on a cruise, they can’t include a provision that they aren’t responsible for anything. Even if the Captain is drunk, sinks the ship, and your entire family are eaten by sharks, you can’t sign away your right to sue for damages.
I know that Libertarians don’t like that, but the reality is that with unequal bargaining power, many people might sign contracts (like slavery contracts) because they are in a poor position in life and sign against their own long term best interests. Government nanny state? I think if they outlawed ALL bad contracts then yes, but to reserve it for a few things is fine with me.
In short, to answer the OP, no. You should not have a right to enter into a slave contract.
Libertarians don’t necessarily dislike that.
Since the OP hasn’t returned to flesh out his argument, it’s unclear that he is talking about a government enforced contract.
You can’t be serious! ALSO from the OP is the following!
and
and most directly relevant to what you said:
A couple of weeks ago I said someone had inflicted the worst case of misunderstood out-of-context quotation on me that I’d ever experienced. Well, you’ve already beaten that record!
Well, I’m asking whether I should be free to do something, so it seems like government enforcement is pretty relevant. What would it mean to say I’m “free” to do something if it doesn’t imply that the government can enforce that freedom?
Anyway, the OP is just a question and some thoughts relevant to the question, it’s not an issue I’m prepared to make an argument about.
There’s a difference between a government enforcing something and the government not preventing something. The government will not prevent you from being a slave if you want to.
Is there any other kind? I mean, sure, anything can happen if all parties agree to do it and nobody finds out, has a disagreement and/or calls the cops or goes to court. You can get away with murder if you don’t get caught. 
But I thought that implicit in a question about whether you would be “free” to enter any type of contract would be if the government would/should enforce the same.
If you really want to be a slave, why do you need the government to enforce the contract?
It’s just like marriage (in more ways than one :)). You are free to get married, but the government doesn’t force you to stay married.
I guess I’m having a slow day because I’m not seeing the difference. Before no-fault divorce laws, the government did force you to stay married if your husband/wife didn’t do anything wrong. Today, though, implicit in a marriage contract is that we will be together until death do us part OR irreconcilable differences. A slave contract would presumably not have such an out, or else I have a different definition of slavery than you.
I was viewing a slave contract as one that was binding upon both parties, giving rights to the master as well. So, if the slave wanted out, he wouldn’t be permitted as per the contract.
Well, let me ask you this: Are you free to get married?
If the answer is yes, then you’re free to become a slave.
What is the dollar value of the lives of your family? All the air you’ll breathe for the rest of your life?
The OP is looking at the issue wrong. Slavery wasn’t employment for no wage, it was humans as property. What is legal or illegal in terms of employment contracts, I have no idea, but there is no way for a human to be property any more, ergo you can’t be a slave.
Truer words were never spoken. 
And they are both done in the same contract.
I think that this is correct. Let’s say that the neighbor wanted me to work for him all summer. In exchange, he agreed to keep my fridge stocked with food, paid all utilities, the mortgage and taxes on my property. Good deal? Maybe/possibly?
Well, that’s what slaves pretty much got as compensation. They worked, but also had all living expenses provided for. The difference is that they weren’t free to get out of the slave relationship, had no rights as people under the law, and could be beaten at the will of the master.
Those last three are and should be absolutely unconscionable contract terms today.
Well, not exactly. The slaves in the US South would typically have to build their own housing and grow their own food and even make their own clothing. Anything the owner “provided” was something that another slave had created. What they had, they worked for themselves. What the owner contributed to the relationship was … um …
Everything slaves had may have ultimately been collective slave produce. It should be obvious with a little reflection that each individual slave didn’t normally do all those different things for himself.
But consider this. If you are employed full-time, isn’t everything “provided” for you by your employer in fact ultimately the produce of other employees?
Well, maybe not entirely. But it seems that any exceptions arguable for the employer may apply to the slaveholder as well.
Theoretically speaking, they offered employment (i.e. a goal in life) and acted as a leader. As you say yourself, the group was self-supporting. That requires leadership and organization. Individuals can be self-supporting via farming and hunting, but it’s much harder. Had the same people grown up as tribesmen in Africa, they’d have been in much the same situation, most likely. They would have been living in crude huts that they built themselves and bossed about by some chieftan who had done nothing in particular to deserve that position beyond having the right parents. And I suspect that if you pissed off the chief, there was some form of punishment for it, and I wouldn’t be surprised if that was often lethal.
Where it becomes a “bad” thing is that, if they had been in Africa, there would have been no other life available to them, so you can’t really say that they were being mistreated. But they were in the US and in the US, they could have become doctors, writers, engineers, or whatever else. There were other ways of living, that as individuals they might have preferred.
In those terms, the question is, should I be free to make myself the property of another human being?
It’s not a question about whether this is possible given current laws or whatever–it’s a question about what the laws should be.