At that stage, these machines are potentially “stuck” on the last compatible major release forever. As modern machines become more powerful, there comes a point when older machines simply aren’t up to the task anymore. MacOS is designed first and foremost for Apple’s latest computers. These upgrades happen once per year, normally around October, and they have a new name like Catalina, Mojave, or Big Sur. Apple has no official written policy on what happens when your Mac is no longer supported by major operating system upgrades.
