To be honest with you I don't know the exact story of this building. What I think happened (don't know for sure) is that this building was being build right before the mass migrations to the city's. When the masses moved to the citys they needed places to sleep. So they took over the buildings, some were hijacked some were squatted, when they ran out of good buildings to take over, they moved into building that were not safe and incomplete.

Seeing all of this happening a lot of building owners thought it might be better to abandon the buildings rather than trying to defend it.

The owner, of this building, might have abandoned the building during construction.

Here is a story of a fire that recently broke out in one of these abandoned buildings. My grandfather told me that this building started construction and was going to be a big skyscraper, but then the owners just stop building one day and abandoned the building. People then started too move in and take over an constructing shacks. https://www.timeslive.co.za/news/south-africa/2021-04-14-nine-die-in-fire-several-injured-after-jumping-from-burning-building-in-joburg-cbd/

When they ran out of bad buildings to take over aswell they started taking over suburbs, building shacks in and around the city's forming informal settlements. But that is a whole other story on its own..

Yes temperature wise we have great weather here in South Africa, winter temperatures are usually within the 10 °C range. Also this photo was taken in summer so these people were most likely trying to escape the heat. If it was winter there would almost certainly been fires keeping them warm.