But it wouldn't happen. Firstly the Apartheid government, while terrible, still tried to present apartheid as a respectable political ideology. They want to be accepted by the rest of the world, reintroducing slavery and becoming a pariah state was never part of their agenda it would basically be commiting political suicide, so they would never reintroduce slavery.
Second, the racists had "evoloved" beyond such crude forms of racism. Apartheid was an attempt to make racism not seem like racism, it became an ideology centered on the notion of seperate development, there is no room in their ideology for slavery.
Third, crude forms of economic exploitation, some carried over from the days of slavery, still persisted in South Africa. Migrant labour, the dop system,the mining compounds, domestic work, etc, were all forms of labour that were not that different from slavery but the government never called it slavery since they painted them as legetimate forms of labour that "benefitted" blacks. So already you have forms of economic exploitation that make reintroducing slavery redundant.
There is Zero chance that any apartheid government would reintroduce slavery. If people still believe that it's possible, I want to know where you get the stuff you are smoking because I want to try it.