Apple keyboard compatibility with 2020 27-inch iMac

None of the Apple keyboards that have Touch ID sensors. Those are only officially compatible with Apple-Silicon-based Macs. Look for the extended and non-extended keyboards that don't have those sensors.

Mice & Keyboards - Mac Accessories - Apple

Note that Apple has a bad habit of setting unusually high system requirements for mice, keyboards, and trackpads. E.g., the Magic Keyboard (USB-C) doesn't support anything before macOS 15.1 (Sequoia)! When Apple released it, they discontinued the previous non-extended keyboard.

This ratcheting of system requirements creates a trap where if you have an old Mac, you might not be able to get a replacement Apple-brand mouse or keyboard when your current one wears out. Or when you grow tired of looking at a keyboard where half of the key labels are missing. Third-party keyboards and mice tend not to suffer from this problem.

Any wired or bluetooth one should work. Note that TouchID only works with certain Mac's

>Magic Keyboard (USB-C) doesn't support anything before macOS 15.1 (Sequoia)

I had to look that one up for fear the usbc keyboard would be a brick -- what the issue is

I think the USB-C keyboards work as a keyboard on older macOS and hardware -- except it needs 15.1 or greater to use the touchID key (and possibly Function keys)

further, there is more to the compatibility issue with Apple Magic Keyboard 2 with older OS/Hardware -- lucky there are a lot of pre-owned keyboards out there I will not have to worry about running my ancient systems for a few more years...