I still think there is a difference between slavery as described above in the picture and real slavery.
forced to work without pay under threat of violence and unable to walk away
That's a possible definition, but it's not really slavery.
For example, being forced to work with pay under threat of violence without a choice is not much different either. (And no, I am not referring to so-called "wage slavery" here. Wage slavery is a left-wing concept that dilutes the meaning of slavery even more.)
Slavery as I meant it is the concept that human beings can be owned by other human beings, that human beings are not free by default or not free forever.
While forced labour and human trafficking a horrible crimes, they are crimes and acknowledges as such by the perpetrators. The criminals in question do recognise a right to be free, they just ignore it. This makes a huge difference. It means that there is no automatic support in society for the crime.
But slavery in Mauritania and Sudan (and even to some extent in Iraq) is accepted by society, not only in those countries but world-wide because the regimes in question are independent countries with the right to do whatever they want in their territories. (Many a time did I wish that Sudan's leaders were Jewish so that the UN would take a much closer look at everything they do.)
Technically, the world does not accept as legitimate what those countries do, but the world does insist that it would be illegal to stop them. But a right that is illegal to enforce is not a right at all. And hence African slaves in those countries remain rightless.
Indentured servitude is an entirely different story. It's not right to mix it in with slavery at all. Indentured servants live horrible lives but they have (technically) chosen their fate, they do get paid, violence is used only to enforce the "contract", and they do get to leave after a number of years. And most importantly, their children do not inherit their status.