Let us suppose for a moment that an adult who was of sound mind and body consented to become a slave and the property of another person.
Do you believe a person can voluntarily become a slave? Do you believe there are any circumstances where it is morally right or rational for a person to do so? Or do you claim that there is some authority to prevent a person from doing so? Does such authority or principle extend to limiting the freedom, rights or choices of adults elsewhere?
I would say no and certainly according to the medieval canon lawyers who pioneered the idea of individual bodily self-ownership, this would have been an impossibility.
Why?
Because of the principle of
persona libera non potest obligari (a free person cannot be obligated) which means that a free person cannot be subject to an obligation that treats him or her as a "thing" rather than a "person".
See:
https://books.google.co.uk/books?id=CAA4GBnA0hoC&pg=PA94&lpg=PA94&dq=voluntary+slavery+canon+law&source=bl&ots=yyg5Iqd6a0&sig=GJPOl4Dn0sGxbMF-zAdUKyxJa3w&hl=en&sa=X&redir_esc=y#v=onepage&q=voluntary slavery canon law&f=true
"...The real intention of the clause persona libera non potest obligari...is to argue that a free person cannot be subject to any obligation that will treat him or her as a thing instead of a person. [This] would transform a personam liberam (free person) into a res (thing). In canon law (as in civil law) this would be untenable..."
To submit yourself voluntarily to slavery is, in a sense, to deny your very personhood which bestows your right to choose in the first place, because only persons have freedom of choice and autonomy. That is a philosophical absurdity, according to the medieval canonists and I agree with them.
You can't freely submit yourself into a state of slavery and thereby transform yourself legally into an object or thing rather than a person because this would be denying the very freedom of choice and personhood that you exercised in the first place to do this, to freely decide.
So morally and philosophically speaking, while people did do this in past civilizations, it is a logical impossibility to reduce oneself into a condition of voluntary slavery...I mean, that's essentially saying "
I, as a free person, do solemnly de-personize myself and have chosen to become a thing, that is an object rather than a person".
Eww.
If you claim to have the freedom and autonomy to voluntarily do things, then you are implicitly recognising that you are a person and persons are not 'things' - so as a person you can't turn yourself into a thing by exercising the very freedom and autonomy that makes you a person, surely?