The situation in South Africa ever since the Group Areas Act of 1950 was that racial groups were restricted to different residential areas. The result was that if a Kingdom Hall or Assembly was in a white area the only people living in that area would be white. Likewise, in a black area the only people living there would be black. The effect of the Group Areas Act was that the congregations were not mixed. There was an exception to this when an employee (usually a maid or gardener) lived on the property of their employer, in which case they attended the congregation in that area.
Ever since the Group Areas Act was abolished in 1991 the congregations were mixed. Although I would say that there have been Witnesses who were racist due to their background, as a group they have never practiced apartheid in South Africa.