The moment your hardware no longer supports the latest version of macOS, it's technically outdated. Apple's support cycle allows for a three-year period where the company will continue to provide security updates for older versions of its operating system. Once that three-year period expires, you're out of luck. At this point, your system can still run the version of macOS presently installed on it, but it will be less safe and pleasant to use over time. It's more feasible at that point to use a different operating system.