Slavery is actually quite widespread throughout the Tortallan Universe and is featured as a major social issue in several books.
Slavery was already a problem for the Provost's Guard around 246 HE before it was forbidden as illegal slave auctions were held in the attempt of cheating the Crown out of their taxes. Furthermore the kidnapping of children for selling them as slaves was already prohibited. In the Lower City of Corus the Market of Sorrows was a known place for slave trade.
Slaves in Carthak have no rights and there are very few Carthaki laws that protect them from brutality. The whipping and killing of slaves is legal if a free person feels threatened in any way. Carthak used slavery as a way to subjugate new conquered lands and often transported conquered peoples to become slaves in other parts of the Southern Lands. Slave catching was sanctioned by the government in the early part of the 450s, and was allowed by Ozorne Tasikhe in order to terrorize Tortall.
Veralidaine Sarrasri petitioned the new emperor, Kaddar Iliniat, to think of the slaves and to free them or help them. Kaddar said he would but that just freeing the slaves would be tricky as many people would rebel against him in fear of slave uprisings.
The indigenous people of the Copper Isles faced enslavement in order to subjugate upon the successful Luarin Conquest. The Isles have no protections for slaves under luarin law, especially raka slaves. Luarin slaves are provided with a bit more assurance due to the color of their skin. Brutal jurisdiction kept even free raka subjugated, such as the hateful and brutal Conqueror's Laws.
Before the Luarin Conquest, the raka did practice slavery, and often enslaved beaten members of warring clans.
There are fewer slaveholding countries that are portrayed in the Circle Universe as opposed to the Tortallan Universe. There are a few countries where commoners could be considered as some kind of slave—or a serf—due to their lack of rights and autonomy in law, such as Namorn.
Evumeimei Dingzai was sold to slavers by her mother, who was disappointed that she wasn't a boy.
Evvy was brought to Chammur and sold. The penalty for a runaway slave was quite harsh. Anyone who found a runaway slave could bring them to Justice Rock for a reward. It is not likely that there are many laws in place for the protection of slaves from the wrath of their owners.
As Chammur is a part of Sotat, it is likely that the kingdom itself is a slaveholding nation itself. If there is no slave trade, however, the prison complex and sentences could be considered slavery, as criminals have no rights once they are sentenced to physical labor for life. This slavery is inflicted on even very young minor criminals, like Briar Moss and a few others of his gang.