Want to use an iMac as an external monitor? You need an old version of macOS

A limited range of iMacs from 2009 to 2014 can be used as external monitors, but only when running certain versions of macOS.
iMacs 2013
Apple

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Glenn Fleishman’s most recent books include Take Control of iOS & iPadOS Privacy and Security, Second Edition (covering iOS 15/iPadOS 15), Take Control of Your M-Series Mac, and Take Control of Securing Your Mac. In his spare time, he makes Tiny Type Museums. He’s a senior contributor to Macworld, where he writes Mac 911.