Zidane wasn't sacked, in fact he quit twice in total as far as I can remember. First he quit while he was still winning, but they started to do badly without him so he came back. And after three years in a row UCL titles, he quit once more. Zidane wasn't fired by anyone, that dude did better than any other coach in that short span of time, so there was no possible way that he would be fired.
You are right. As far as I know, he didn't quit or was fired. Zidane didn't want to extend his contract with Real Madrid. It is difficult to understand his mindset, he is a typical of unpredictable coach. He has a perfect career as a coach with Real Madrid, he may feel satisfied and doesn't want to continue it. He may think to get a new challenge but he doesn't manage any team until now.
I am not expecting him to come back neither, there is no way that he is ready for that. He talked about how hard it was and he just wants to spend time with his family instead and that's why he quit. We need to remember, these players travel a lot and spend time at training grounds a lot, so that means they are seeing their family very rarely, that's not ok by some of them.
I actually expects him to return as a manager but Zidane always prioritizes his family. So, he may think it is the time for family, he is no longer interested to continue his career as a manager. Until now, I never heard that Zidane will have a plan to come back soon. It means Zidane probably has no idea to keep managing any club. Well, we must appreciate him, it is his own decision.