Want to use an iMac as an external monitor? You need an old version of macOS | Macworld
Review: 27-Inch iMac 5K with i5 processor - 2012 on the outside, 2019 on the inside | AppleInsider
How to Connect iMac to Monitor | use external monitor as a second screen for iMac Pro - YouTube
Apple iMac 5K 27-inch (2019) Review: Looks like 2012, Performs like 2020 | Digital Trends
High-End Mac Showdown: Apple iMac vs. Apple MacBook Pro | PCMag
How to Use an iMac as an External Monitor
That question again: iMac or external display? – Six Colors
How To Use an iMac as a Monitor for a PC
Apple iMac 27-inch Review (2019): Full Review and Benchmarks | Tom's Guide
Using iMac As Monitor Requires Very Specific Cable | Cult of Mac
Identify your iMac model - Apple Support
Bloomberg: New iMac with Pro Display XDR design coming this year, cheaper external display also planned - 9to5Mac
Apple iMac 21.5-inch (2019) review | TechRadar
Apple Studio Display: Everything We Know | MacRumors
iMac (21.5-inch and 27-inch, 2019) – Full Information | iGotOffer
Incase anyone ever needs the answer…I have a 27” 2010 iMac working in Target Display Mode with a M1 MacBook Pro using a miniDP to USB C adaptor that's capable of 4k60Hz.