This lore can be found on Korriban.
X: -418, Y: 46
This can be found on the second floor of the Sith Academy. It is located on a shelf on the upper level of the room where the Sith Warrior skill trainer is.
In contrast with the Republic, which officially condemns the practice, slavery is widely accepted across the Empire. However, instead of slaves being bought and sold merely on an individual basis, the custom has been fully incorporated into the hierarchy of Imperial society. As the lowest caste, slaves perform menial duties and backbreaking labor, and they possess virtually no rights or recourse under the law. The ranks of slaves are typically made up of those who fall outside the traditional Imperial power structure: prisoners of war, criminals and aliens from conquered worlds. Children of slaves are born into the same caste as their parents, though it is possible for individuals to be elevated should they show strength and ability that would better serve the Empire in a more distinguished role. Recently–seeking to replenish the ranks of the Sith Order after the war–even the Sith Academy has started accepting slaves who show a strong affinity for the Force… though many still look on these former slaves with disdain.