He was released in early 90s. Remember the other anti-aparthied stuff that was going on at the time? The sporting sanctions, trade sanctions and all. I do. I remember the period well. You are a little young to have any valid memory of the period, so what you know is what you have heard. No, we did not think he died in prison at the time. There was a song, Free Nelson Mandela, that was very popular in 1984.
The Mandela effect is an excuse for bad education and poor memory, nothing more.