IIRC, in the OTU slavery was against Imperial law. But what "slavery" was in the OTU, and in any other society varies by context. The thread has, I believe touched on all the possibilities:
A. Legal Ownership, in the entirety, of another sophant, who has no legal rights at all, who had no input into their status.
B. Legal Ownership, subject to limitations in time, of another sophant, who has no legal rights at all, who had no input into the transaction.
C. Legal Ownership, in the entirety, of another sophant, who has no legal rights at all, who consented to the transaction creating their status.
D. Legal Ownership, subject to limitations in time, of another sophant, who has limited legal rights, who consented to the transaction creating their status.
E. As A., but limited to a single, geographically isolated legal jurisdiction.
F. As B., but limited to a single, geographically isolated legal jurisdiction.
G. As C., but limited to a single, geographically isolated legal jurisdiction.
H. As D., but limited to a single, geographically isolated legal jurisdiction.
I. De facto (and illegal) ownership of of another sophant, induced by fraud.
J. De facto (and illegal) ownership of of another sophant, initiated by force.
K. Coercive relationships which, while fully legal, limit the effective exercise of legal rights by a subject sophant to the extent that free will is overcome, and a state equivalent to I or J exists.
L. Coercive relationships which, though legally shady, limit the effective exercise of legal rights by a subject sophant to the extent that free will is overcome, without outright fraud or force, so that a state equivalent to I or J exists.
IMTU, which I try to model on the OTU, only H & K are permitted on member worlds of the 3I. I & J are actively enforced against by 3I's law enforcement, to include customs and immigration officials.
There are other legal possibilities, such that the initiation and transport was elsewhere illegal, but in protected by the resident jurisdiction. Some, such as K and L, are subjective as to what degree of coercion is required, and what the dividing line is between "legally shady" (e.g. holding passports until "early departure penalty" is paid) and outright fraud (e.g. contracts for "Indentured Nanny in Regina," and ends up in a brothel in Louzy).
Similarly, any punishment is, to some extent involuntary. Also, they are to some extent voluntary, if the conduct was clearly illegal and the possibility of punishment was known. Clearly, legal systems that create ex post facto laws can demolish this distinction.
In sum, where and when it happens, some "slavery*" is a clear crime (I & J), some is legal (A-H). The former will always exist, to the extent that crime will always exist; more in some contexts, and less in others. The latter will vary as widely as the legal landscapes exist, and will often be considered indistinguishable from the former by members of other societies.
Just my Cr.02
*I use quotations not to minimize dispute this as what laymen would call slavery, but to indicate that there are many other terms used in penal codes and civil law, (such as "abduction," "assault", "rape", "false imprisonment", etc.) that would cover a single period of such illegal slavery.