What iMac model is not compatible with Big Sur update?

Anything older than the Macs listed below are not compatible with Big Sur.

These Mac models are compatible with macOS 11 Big Sur :

MacBook (2015 or later)

MacBook Air (2013 or later)

MacBook Pro (Late 2013 or later)

Mac mini (2014 or later)

iMac (2014 or later)

iMac Pro (2017 or later)

Mac Pro (2013 or later)

For details about your Mac model, click the Apple  icon at the top left of your screen and choose About This Mac.