Slave -- not sure. In real life, what you describe wouldn't count as consent. Might in COE, we don't know yet. Also, it is not clear that "death" will be an available penalty for an express contract. I rather think it won't be. Also, service contracts may need to have a defined and limited term -- and "for life" might not be permissible. Service contracts also may need to have a limited and specified description of the the service (scope of work). On the other hand, if things like religious vows or oaths of fealty are to be handled by contract, perhaps lifelong, general service obligations should be allowed.
In any case, penalties for breach of contract don't happen automatically and there will probably be a process for limiting the effectiveness of bounties. If you capture someone in a war with a neighboring kingdom and are able to enslave them (big if, as sparks survive death and souls survive sparkdeath, so threat of death isn’t that strong a motivator), they probably could escape back to their home kingdom, where the contract would not be recognized as lawful. Depending on how the bounty system works, bounty hunters may still be able to try to capture them for the bounty -- but there should also be the possibility of making that an illegal bounty, meaning the bounty hunter would be committing a crime by trying to capture them.
If there is slavery, I would think it is more likely not something agreed to in a specific contract, but rather imposed as a result of something. Perhaps nonpayment of debt. Perhaps capture in war (based on the war terms of the combatants, which might also be some kind of contract (the contract system may be versatile enough to use to set war aims and victory conditions, and, if so, it could also set the consequences of defeat, which could potentially include slavery of captured combatants I suppose)).
That said, I'm sure slavery can be roleplayed if people want -- so contracts would only be used if one of the people didn't want it, and that is problematic. On balance, I hope contractual slavery isn't a thing, but I think it might be too hard to set up contracts such that something like this, at least for a term, can't be approximated, so a prohibition might not come from the game mechanics, but from TOS or some other meta-intervention.
Indentured servants -- That is like slavery light or slavery until some other obligation is discharged. There are problems with that. For instance, depending on the level of control you have, you could prevent the person from ever paying the debt or discharging the obligation. That would make it effectively slavery. As with slavery, I hope that that isn't a thing -- but it might require some meta-intervention to prevent. That said, debt contracts should be a thing -- and repayment by work makes sense as well.
We know employment contracts will be a thing for both PCs and NPCs. We don't know how that will work because the example contract we were given was not an employment contract. It probably should work as payment for defined services or labor. If payment is in advance and the work is not done, that would be a breach entitling the employer to refund. If work is done in expectation of payment and wages are not paid, that would be a breach entitling the employee to collect wages. Both could be the basis of a bounty, and, if collection costs are part of the damages or penalty, those could be added to the amount due.
Term -- that sounds like slavery or indentured servitude for a defined time period, with the same problems. This can be role-played. That said, apprenticeship contracts often worked like that historically, and that might be something worth having.
The only game enhancement I see from the slavery contracts mechanic (or slavery light contracts mechanic) is that it could engage the bounty system to enhance master/slave roleplaying by adding a "slave catcher" bounty hunter element. I'm really not sure that that is a road SBS should go down, or should want to go down. If this is allowed, expect Gorean roleplayers to migrate over from Second Life.