Target Display Mode completely dead in Mojave and later?

The reason I was troubleshooting this was because our workplace purchased new iMacs for some of their employees to use at home - strictly as work computers. A number of the employees have older personal iMacs that they wanted to be able to use as second monitors (largely for space considerations). So my role in this is really a support role, not a personal one.

Turns out the "mystery" above was solved by an SMC reset on the 14,2 iMac. So that model can be put into Target Display Mode now. It works fine with the 13,2 iMac - in either direction, despite Apple's documentation stating that Target Display Mode was dropped in Mojave.

So this confirms that my initial understanding was correct - that Apple dropped HARDWARE support for Target Display Mode, but I don't think it was as long ago as 2014. Clearly it is still supported on Catalina, if you are running it on an older iMac.

I have 3 iMacs, models and OS's listed below.

I may have answered my own questions. The only combo that works is in bold

#1 - iMac 20,1 (5K, 27-inch, 2020) with Catalina

• Cannot enter TDM mode with either #2 or #3 as source (I didn't really expect it to)
• Does NOT work as source for either #2 or #3 using Apple TB3 to TB2 adaptor. I did expect this to, perhaps over-optimistically

#2 - iMac 14,2 (27-inch, Late 2013, with Mojave)

• Cannot enter TDM mode with either #1 or #3 as Source
• Works as a source using #3 in TDM mode

#3 - iMac 13,2 (27-inch, Late 2012 with Catalina)

• Enters TDM mode using #2 as source

I think the only actual mystery is why #2 doesn't go into TDM mode with #3 as source. I could try taking #2 to Catalina and see.

So yes, clearly the Apple documentation is not correct when they say TDM is only supported through High Sierra.

#2 turns out to work as a TDM display - it just needed an SMC reset. Mojave.

#3 was definitely running Catalina.

So yes, Apple's info is not correct. It's not the OS that determines if the iMac can be used in Target Display Mode - it's the vintage of the hardware. From what I've read, the introduction of the 5K monitor in 2014 is when the feature was dropped from hardware.