Most Mac users wouldn't dream of replacing macOS with anything but the latest update, but Google wants you to at least consider another option: Chrome OS.
Chrome OS Flex is a free and sustainable way to install Google's online OS on any Mac or PC. It's built for schools and businesses looking for modern computing with cloud-based management. It offers the same Chrome OS interface as you would get on a Chromebook with fast access to VDI and web apps with an intuitive, clutter-free, and reliable experience.
That makes it perfect for an old Mac that can't properly run Big Sur or Monterey, but it's even more useful for small businesses or cash-strapped schools looking to build out a fleet of machines. Google says Chrome OS Flex can be installed in a few minutes and has been tested on Macs as early as the 2009 MacBook, though early models are likely supported as well as long as they have a working Internet connection.
To get started, you can sign up for early access on the Chrome OS Flex site. After registering, Google will provide instructions for how to create a bootable USB drive. Chrome OS Flex can either be permanently installed on your Mac or run from the USB drive.