There is a lot of posting in the internet...
Yes, its lovely isn't it! (not), Rumors of TDM's being not dead are either exaggeration or post-processed horse fodder.
There is even confusion among us old graybeards here due to the poor presentation of Apple's now-archived support article on Target Display Mode. Being archived, it will get no further updates or clarification. That leaves organizing the information and finding the current situation to us, which resulted in this:
VIDEO SOURCE COMPUTER (the one sending the video signal to an external monitor):
- A 2019 or older Intel Mac running macOS 10.15 Catalina or older.
- Apple Silicon Macs are not supported
- mac OS11 Big Sur is not supported.
TARGET COMPUTER (the one to be an external monitor):
- iMac 27-inch (Late 2009 and Mid 2010)
- any iMac from Mid 2011 to Mid 2014.
- The target computer must run no higher macOS than OS 10.13.X High Sierra.
Apple started the deprecation of TDM in late 2014 (😳) and threw the last shovel of dirt in its unresponsive face in early 2020. It is a dead issue. You can spend up to US$100 on thunderbolt cabling and adapters and still not have TDM work. I can buy a really nice external monitor for $100 and use a HECK of a lot less energy than an iMac needs in just being a display.
