South Africa will not be touring Pakistan towards the end of March 2020. They announced it today and cited player workload as the reason behind it. South Africa will be finishing its India tour on the 18th of March and the IPL will start from the 29th of March. So they were left with just a small window of one week to finish the three-match t20 series.
The workload was really heavy as IPL is the most hectic t20 league in the world as all the teams have to play 14 matches before the playoffs. Teams play more if they qualify for the playoffs. Most of the South African players will be playing in the IPL and touring Pakistan for three matches would have added to their fatigue.
Pakistan Cricket Board said that they respect cricket CSA's decision and was optimistic about the future. There hasn't been any date finalized for the three-match t20 series to happen but I think it will happen only after the end of IPL. Pakistan will also be playing PSL which will come to an end on 22nd of March.