When I worked in Windows they had entire teams dedicated to backwards compatibility testing and "sustained engineering". At the end of every release cycle there would be a multi-month effort to pack up all of our test automation and hand it off to another team who owned running it for the next several years. Plus SLAs with giant companies that could get you camped out on the floor of a datacenter with a kernel debugger if you pushed out a broken update. It was never a totally perfect system, but they invested a lot of effort (and money) into it.