Downgrade from Catalina

Thanks for providing the purchase date of your Mac, it helped me a lot!

It looks like you bought your Mac after macOS Catalina was released. The version of macOS that came with your Mac is the earliest version of macOS that can run on your Mac.

If you want an earlier version of macOS, buy a used Mac from a date earlier than the macOS Catalina release data, October 7th 2019. These Macs will be able to run macOS Mojave.

The same applies to other versions of macOS.

FWIW, since you didn't know which version of macOS your MacBook Air shipped with it became a guessing game. It's true you can't downgrade to a version of macOS earlier than what it was shipped with. However, you can install Mojave as a VM using Parallels Desktop, VMWare Fusion, or VirtualBox. How well a VM will run games will depend on the game.

The first answer is correct.

If you purchased your MacBook Air in January 2020, it was still the 2019 model, as the 2020 model wasn't released until March. Therefore, regardless of whether your Mac came with Catalina pre-installed or not, it technically _can_ be downgraded to Mojave (although it is a tad complicated; google it and you will find the proper instructions).

So it is simply not true that _every_ Mac can only use the OS that it came with (and later compatible OS versions, obviously), under certain conditions many Macs _can_ be downgraded to earlier OS versions than what they came pre-installed with.

However, if you (like me) had bought the MacBook Air in say July 2020, you would have gotten the new 2020 model which unfortunately/regrettably _cannot_ be downgraded to Mojave -- you're stuck with Catalina. If I cannot find a way to somehow add the required Catalina drivers to Mojave now, thus "forcing" Mojave to work on the MacBook Air 2020, I will simply return it to Apple (or sell it)... :-(

FWIW, the MacBook Air 2020 wasn't released until March 2020. If you have a MacBook Air 2019 you can revert to Mojave.

See: https://www.imore.com/how-downgrade-macos