I am not sure if people today have a correct depiction of slavery in mind, we might imagine it is always peoples work half naked in chains and get whipped, i personally think slavery is not always so. Maybe there was such thing in the past, maybe they being exaggerated in modern propaganda
You are a slave if you are owned by someone, become a comodity, you can be used as a thing and can be forced to work like animals. So by means, even though you are not get whipped, not being put into chains, you are still a slave if you are a slave. What make you a slave is because you are not free, you are bound to your master.
Maybe your master is nice, a good person, never beat you, never force you, always give everything you need to live such as food, clothings, place to stay...but you are bound to your master and not living free as everyone else who are free. It is still slavery.
I think Tevinter Imperium slavery is like this, not the beating with whip kind of slavery, but slaves are slaves because they are simply owned by their master. Furthermore, with mind control blood magic, why need a whip to force people to work?