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

https://support.apple.com/en-us/HT204592#requirements

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.

How did you make it work with the 2013 with Catalina as a second monitor? I had to downgrade it after I found the above Apple support article.

You may be out of luck. Only the 2009 and 2010 iMac 27" with Mini Display Port can be used as a secondary display to the 2020 iMac 5K Retina. I may be wrong, but your 2013 is too "new" to be attached to the 2020 iMac 5K retina.

All my searching on the topic appears that Apple removed the ability for the mid-2020 iMacs to use TDM with older iMacs that used to be functional with prior iMacs.

There is TDM after high Sierra. Many of us, including myself used even on Catalina. It’s the 2020 iMac that cut support for it.

Sorry, my experience is limited to using the Luna dongle to use my 2011 iMac running High Sierra as a second monitor for my 2020 iMac running Catalina. I have no experience with PC's. However, the Luna webpage (https://shop.astropad.com/pages/choosing-your-luna) does purport to offer a dongle that will enable an iPad to be used as a second display for a PC. If you want more information about this you might contact the Luna folks directly-- support@astro-hq.com . As I have previously stated, I am pretty happy with the performance I am getting using the Luna dongle to enable my older iMac as a second display for my new iMac.

I cannot get this to work with my new Mid-2020 iMac (w/Radeon 5700) connected to my late 2013 iMac. Before I upgraded to the Mid-2020 iMac I was connected successfully to the 2013 iMac in TDM from my late 2014 5k iMac so I know the older iMac can work in this configuration. The new Mid-2020 iMac seems not to be sending the display signal out. I've tried two different adapters. Both the Apple Thunderbolt 3 to Thunderbolt 2 adapter and a OWC Thunderbolth 3 to dual DisplayPort adapter and neither are working. When I press CMD-F2 or Fn-CMD-F2 on the target iMac it never shows the mid-2020 screen like it used to with my 2014 iMac.

Do you have the mid-2020 iMac? What adapter and cable are you using between the two systems?

Thanks! Hope they didn't remove the feature from the mid-2020 iMacs!

I have this problem with my new mid-2020 iMac.

My setup that worked (for many years up until this morning):

iMac retina 2015 -> TB2 cable -> iMac 2012

Getting the new iMac I noticed that it has the small TB3 ports so I went down to the Mac store and bought the recommended adapter: “Thunderbolt 3 (USB-C) to Thunderbolt 2 adapter” (and it was and Apple original, so not cheap!). The new setup is:

iMac retina 2020 -> TB3toTB2 adapter -> TB2 cable -> iMac 2012

But it doesn’t work. I can see that the TB bridge get IP-addresses on both ends so there is definitely some connection between the two computers. The CMD+F2 on the target computer (iMac 2012 with original, wired, keyboard) does nothing. 

Testing with my MBP it does work just fine:

MBP 13” 2018 -> TB3toTB2 adapter -> TB2 cable -> iMac 2012

My iMac is slightly different from the one in the top post:

Are you saying that the new mid-2020 iMacs will not work with the Apple Thunderbolt displays? I just bought two because my mid-2020 wasn't working with my two 21" iMacs I had been using. I sure hope they work with the Thunderbolt displays. If you have any more information on this topic please let me know.

I have not been able to get this to work using my new iMac (Retina 5K, 27-inch, 2020) and connecting to my older Mid-2011 iMac 27". One of the benefits I thought, was that I could still utilise my ageing iMac display, which is perfectly good. I have the Apple Thunderbolt 3 to Thunderbolt 2 Adapter and a brand new Thunderbolt 2 cable, but alas. Ctrl F2 does nothing. ie, Target Display Mode doesn't work on Mid-2011.

Is there some other way to use the old iMac display ? Some kind of Airplay / 3rd party App ?

I mainly want to use it for a 2nd photo editing display (Lightroom second screen).

Any help appreciated.

As they say in "Who Wants to be a Millionaire", is this the final answer here? I have just purchased a 2020iMac running Catalina and I wanted to use my old 27" iMac (late 2012) as a second screen for editing video in Final Cut Pro. I purchased the USB-3 to Thunderbolt adapter and a 3.3' Thunderbolt 2 to Thunderbolt 2 cable. I then discovered that my 2012 iMac couldn't be used in TDM as I had upgraded software to Catalina, so I went through a lengthy process of wiping the drive and reinstalling High Sierra. Having done all that, I connect the two computers using the adapter and the cable and press Command-Function 2 and Voila!! Nothing happens. Has all my effort been a waste? Any possible workaround? (Hoping against Hope.)

Thanks for your prompt and to the point reply. Wasn't what I was hoping for, but I guess you must face reality. I wonder if the folks at Apple might think of reverting to the "good old days" when one could put these older machines to good use since so many seem to want this functionality. I guess, as Nellie Forbush sang in "South Pacific" (a reference that most of the world would not understand today--Google it), I'm just a cockeyed optimist!!

Has anyone any experience with Luna Mac-to-Mac or Duet as a patch to enable screen sharing between an older Mac (mine is a 27" 2012 vintage) and a 2020 iMac running Catalina? These applications claim to be a stand-in for Apple's Sidecar which is intended to allow an older iPad function as a second display. They would appear to enable second screen functionality between new iMac and older iMacs (source monitor running Catalina and the target display running Mountain Lion or later). Any experience to share?

Hello I just came across your post and I was wondering if you could please explain to me how you did this. I have the following set up IMac 5K 2020 I am attempting to use a second iMac late 2013 as my secondary display I have bought the thunderbolt cable and the adapter and I cannot get it to work. From your pictures here I see you got it to work could you please explain to me

Hello everyone

Sorry for my English, it's half done through a translator.

My question is that I don't have 2 iMacs but I have a desktop PC and I would like to switch my iMac 2020 27*5K and use it as a monitor for a regular PC too. The output I saw lunadisplay for Mac to Mac but what about for Mac to PC? I spent money on thunderbolt 3, but then I saw on the forums that it does not work on 2019 and today's iMacs, but is there really no workaround or solution at all? I have a very powerful PC and I would be crazy to use the iMac 2020 27^5K as a monitor with the ability to switch as it was before cmd+F2, please tell me some way I'm sure there are people very advanced in this regard

Thanks a bunch, was hoping Apple was wrong about this. :)