As far as I know, Shannen Doherty and Kathleen Robertson (https://www.reddit.com/r/BeverlyHills90210/comments/crzjht/why_did_kathleen_robertson_leave/?ref=readnext) were the only ones (https://www.nickiswift.com/72885/dark-secrets-cast-90210-tried-hide/) who left at the very end of production. I don't think that we need to delve to much into the circumstances (https://www.nickiswift.com/49507/untold-truth-beverly-hills-90210/) for why Shannen left since i'm pretty sure that it has been covered up and down for the past 25 years.

And if I'm not mistaken, Hilary Swank (https://www.youtube.com/watch?v=9rqz0weszTM) and Jamie Walters (https://www.ranker.com/list/beverly-hills-90210-drama-behind-the-scenes/melissa-sartore) were for whatever the reasons, fired. The real question however is, why exactly did Jason Priestley (https://www.cnn.com/2014/05/14/showbiz/jason-priestley-memoir/index.html), Luke Perry, Tiffani-Amber Thiessen (https://groups.google.com/forum/#!topic/alt.gossip.celebrities/aNC1t0EqoNE), and Vanessa Marcil leave before the seasons were up? Well, I assume that Luke Perry technically didn't resign, he wanted to work on movies. Otherwise he would not have been allowed back.