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

I have sent my configuration to the folks selling the Luna hardware and asked for their confirmation that it will allow a 2020 iMac to us a 2012 iMac as a target display. Their advertising for Mac to Mac suggests this should work. But I suppose the more precise question is whether Final Cut Pro will recognize it as a second linked display and allow someone editing video to put a portion of the apps work displays (for example, the Viewer) on the second display while leaving the browser and the story line on the source Mac. For we don't want it just to mirror the screen of the source iMac--that would not be helpful. We want to use two screens to divvy up the working parts of the program. Does anyone know the answer to this question? This might be more of an "Apple FCP" question than a Luna question. Also, while I have been looking to make some use of my 2012 iMac, I would like to start using its 1 TB hard drive to store videos. This has peaked my interest in Target Disk Mode. This would seem fairly easy to set up once the machines are cabled together, but based on the information that I have seen, using the 2012 iMac as a spare disk storage location with Target Disk Mode would preclude it being used simultaneously for anything else. I suppose you could enter and exist Target Disk Mode and then use the Mac as a target display, but that is a little cumbersome. Does anyone know the answer to this? Since when Target Disk Mode is turned on, the OS of the target disk computer is not loaded on start up, it would seem the TDM computer will function only as a spare hard drive. Can anyone confirm this or correct this assumption?:

Here is the question that I posed to the Luna Display Support people followed by their answer:

I want to use my old iMac (2012 iMac now regressed to High Sierra IOS) as a second display for my 2020 iMac running Catalina when editing videos with Apple's Final Cut Pro. Before purchasing I am seeking confirmation that my 2012 iMac can be linked with my 2020 iMac as a second display and, equally important, that Luna Display 4.0 will not just mirror the screen on my new iMac but will permit me to export just parts of my source iMac's screen to the second display (for example, the timeline in FPC).

You can use Luna to turn your 2012 iMac into a second screen for a 2020 iMac. For Mac-to-Mac mode, the primary Mac must be at least a 2011 Mac with macOS 10.11 El Capitan or later, and the secondary Mac must be a 2007 Mac or later running macOS 10.8 Mountain Lion or later.

The Luna unit needs to be plugged into the primary Mac directly (your 2020 iMac).

More information about Mac-to-Mac mode is available here:

https://help.astropad.com/article/189-mac-to-mac

Once you set up Luna Display, you will be able to drag windows from the primary Mac to the secondary Mac screen. In other words, you can have different programs/windows allocated on both Macs.

There is also an option to mirror the screen of the primary Mac instead of extending it.

Luna can be used for video editing, but please note that it does not stream audio from primary to the secondary Mac. Audio can only be played on the primary Mac.

Let me know if you have more questions!

So, based on this reassurance, I am going ahead to purchase the Luna Display 4.0 Dongle. I will report back after I have run it through its paces.

Lake, I hope this works. I've been attempting to get Duet Display to work on my 2012 - 2020 iMac setup. Duet support has been great but I have been having mixed success in getting it to work. So far I can't get it to work reliably. I would rather spend $80 once than $20/year subscription to be allowed to extend the display wirelessly. It works flawlessly with my iPad and my iMac.

OK--I now have been working with the Luna Display dongle for a few days (using Apple's Final Cut Pro) and I am pleased. Set up was pretty simple (you need to plug the dongle into the appropriate USB port on the back of the computer--I ordered the Dongle with the USB 3 male attachment) and download two software programs--one for the dominant computer (my 2020 iMac) and the other for the second display (my 2012 iMac). Both iMacs have the glorious 27" display. I am attaching a photo of my setup. My 2020 iMac is on the left and the second display is on the right. FCP recognizes the second display right away and allows you to allocate one of more of the active windows to the second display (I choose to keep the timeline on the main computer and allocate the browser and Viewer windows to the second display). As you can see, you get a pretty grand view of your project. I suffered a little confusion in configuring my display (you are asked to choose between two graphics displaying two stylized computer screens to indicate on which side you are placing the second display--I thought the graphic was not terribly clear as to which computer was the dominant display. But once I got that figured out, the program seems to operate pretty seamlessly. Your pointer runs fluidly from one screen to another (you can lose the indicator among the large area of display on two 27" computer screens). I guess consistent with operating the two displays as one giant display, you will find your Apple menus and the columnar program selector display on the second display (if you elect to place your second display on the right side), but it all works pretty well. When you wake your computer from sleep you will need to activate both computers (enter PW) and wait a few minutes for the computers to link wirelessly through your network, but after that you're off and running. Overall I am very pleased. A real good use for an older iMac which still has a beautiful display. I am also using Target Disk mode to use the old iMac's hard drive to back up files. Well worth my investment of $80 (I spent more on cables than this trying to make my old iMac work through Target Display mode).

Something went wrong when I tried to upload a picture file that I sent from my cell phone and Safari crashed. I will post this reply and then try to post my picture.

Attempting to post the .jpg

OK, Safari did not crash--but I got a notice that there was an error trying to upload my image and to try again later. I will do that. At any rate, this does not appear to be a problem with Luna Display. Either the type of file that I am attempting to upload or some problem with the Apple website.

After lots of time online and on the phone with Apple, confirmed that it's a software change not hardware: no TDM after High Sierra. If anyone knows of a patch/workaround, please post.

Thank you! Now I just have to figure out why it's not working on my 2017 iMac. Maybe it's because I'm using a Thunderbolt 2 to 3 connector.

I'm trying to use a MacBook Air as the source and the iMac as the display. Sounds like that's not possible.

The 2020 iMac cannot be a source for a thunderbolt based iMac (receiver).

My understanding is that Zahilla used 2020 iMac as source.

As melodicskys says, it worked for MDp, (27" 2009 & 2010 iMacs), not 2011 to Mid 2014 iMacs requiring Thunderbolt.

Note: Apple said iMacs older than 2010 will NOT support TMD with a 2020 which explains the success of the original poster.