Display

You can connect any of these older iMac models to another Mac from 2019 or earlier for use as an external display. Learn how to identify your iMac model.

• 27-inch iMac models introduced in 2009 and 2010: Use a Mini DisplayPort cable to connect your iMac to a Mini DisplayPort or Thunderbolt or Thunderbolt 2 port on the other Mac.
• All iMac models introduced in 2011, 2012, 2013, and mid 2014: Use a Thunderbolt cable to connect your iMac to a Thunderbolt or Thunderbolt 2 port on the other Mac. If the other Mac uses Thunderbolt 3, you can use the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter with your cable.

Target display mode requires macOS High Sierra or earlier on the iMac used as the external display. It does not work with macOS Mojave or later.

I've seen at least one post that swore OSX above 10.13.6 worked fine, cannot confirm.

You'd have to erase the drive & conflicting reports if an older OS can be installed after Catalina install... maybe boot from an external drive?

https://itunes.apple.com/app/macos-high-sierra/id1246284741?mt=12

The 21.5 inch iMacs do not support target display mode (TDM) from other Macs.

Great to hear, thanks for the report! :)