Tested, iMac 2020 STILL supports target display mode with older iMacs, advice for other people:

I can attest too that this configuration (2020 iMac 5K Retina with 2010 iMac 27") works in Big Sur 11.2. You do need the Mophie USB-C Cable with Mini DisplayPort Connector

I just set up my 2020 iMac with Big Sur and it does not work with my 2013 iMac on Catalina

I previously had a 2018 iMac (which I upgraded to Big Sur) and it was still working

After many calls, I was told that the 2020 iMacs will no longer be able to use an older iMac as a target display

:(

I am so upset! after spending all that $ ...I wish I would of known first

If anyone can tell me other wised, please let me know

The reason it is likely working with the 2009 iMac is that

it is a Display Port iMac and not a Thunderbolt. Even

these 2009 iMacs with a little effort could work with

Display Port PCs but no Thunderbolt iMac will ever

work.

So, it is likely that it is much simper to update an "archived"

document with a simple one line statement saying nothing

works rather than a lengthy Display Port vs. Thunderbolt Port

explanation.

As of yet, no one has chimed in about getting a Thunderbolt iMac

(2011- 2013) working with a 2020. This also one fact that has me

hesitating about pulling out my credit card for a new iMac. The

other is the desire to get a Nikon D500 with 16-80mm kit. Both

end up costing the same.

If your 2010 iMac is 27" it should work in Target display Mode with a 2020 iMac, despite what Apple says. It would need a USB-C to Mini DisplayPort cable.

mophie USB-C Cable with Mini DisplayPort Connector - Apple

Check out Luna Display--www.lunadisplay.com. I have a 2020 iMac and an older 2011 iMac and wanted to use the old iMac as a second display for video editing. Luna Display is a dongle which you can plug into a Thunderbolt input on your new iMac which then uses your wireless network to allow your old iMac to function as a second display. You can drag windows to either screen and your cursor moves fluidly between the two displays. For $80 it is well worth the price. I have been using it for a month and am pretty pleased.

Duet Display support was able to help me configure my setup to have the 2012 iMac work as a secondary display. As I mentioned in my previous post. You need to subscribe to the Air level of functionality. The that subscription is $20/year. Make sure you have the same version of Duet display running on both machines. Launch Duet display on both machines and then on the secondary machine on the left hand side under connect select display and then your primary mac should show up. You then have the ability to mirror or extend your primary display. And that is all.

I hope this helps.

Paul

Bottomline is that 2020 iMacs will work with any Display Port

or MiniDisplay port supported display. It will "should" work

with a Thunderbolt3 display.

The issue is that Thunderbolt1/2 iMacs in TDM are not really seen

as a Thunderbolt "display" but as sort some hybrid which requires

requires hardware/firmware functionality on the host to make it

work which has been removed in the 2020 iMacs and also why even

Thunderbolt PCs cannot work with iMacs in TDM or even the old

Apple Thunderbolt Display.

I can confirm that the new mid-2020 iMacs work with the Apple Thunderbolt Displays. I received two used ones today, and plugged them in and they worked without issue. Unfortunately, the mid-2020 iMac does not work with older iMacs that support TDM. This is somewhat a bummer because I actually liked the old 21" iMac better for my 2nd and 3rd displays than using the 27" Thunderbolt Displays. The screens were less reflective, the were a better size and they didn't have backlight leaking issues that the Thunderbolt Displays have.

I have the same issue: my 2020 iMac 27" is unable to use either of my older iMacs (2010 or 2017) as external displays. Apple informed me the 2020's can't connect to the older Macs. I found this workaround that is a fairly good substitute. https://www.sharemouse.com

LakeWatch, I'm trying the exact same thing. Target Display imac = 2012 27" imac (second screen) and 2020 27" imac as main screen. I've currently purchased the TB3 to TB2 dongle and a thunderbolt 2 cable. The old 2012 imac is currently running Mojave and I will be installing/reverting back to High Sierra on that mac. It is my understanding that the 2012 in that configuration should work as a second display. I also have a 2015MBP to test this on as well. I am. hoping that I can get it to work with my 2020 imac.

I should say the 2010 iMac 27" is stuck at the High Sierra OS as that is as high as it can go as far as Apple's automatic OS upgrade goes. Good thing too, as later OS may not be supported for Target Display Mode.

Thegilfan, check out DuetDisplay. They have a Windows client.

I hope this helps.

Paul

Use your iMac as a display with target display mode - Apple Support

The 2017 iMac can't be used as a display but it should work as the source for Target Display Mode. What iMac are you trying to connect it with?

Correct, 2014 was last iMac that could be used as as a Monitor.

I haven't tried it, but Turn any Mac or iPad into a second display...

https://lunadisplay.com/

Review by NAK005, you may not be happy with it…

https://discussions.apple.com/thread/251259038?answerId=252422503022#252422503022