[arm]

iMacs actually used to (models from 2009 to 2014) support being used as a monitor for external input:

“Use your iMac as a display

You can view the desktop of your Mac on the display of some iMac models using Target Display Mode. In some cases, you can also use Target Display Mode to play the sound from your Mac (called the primary Mac) on the speakers of the external iMac. For example, if you have a MacBook Pro you could use an iMac as the display and for playing audio.

Note: Target Display Mode isn’t supported on iMac models with Retina display. Only iMac (27-inch, Late 2009), iMac (27-inch, Mid 2010), and iMac (Mid 2011 to Mid 2014) support Target Display Mode.”¹

――――――

¹ — https://support.apple.com/guide/mac-help/mh30822/

[rtkwe]

Neat, wonder why they stopped. Sounds like the Retina models might have too high of a resolution to go over a single Thunderbolt connection but it should still be able to do a scaled up resolution...

[Psyonic]

I don't have the answer to why they stopped supporting this, but I don't think it's because the resolution is too high for a single Thunderbolt connection. I have an LG 5k that works over a single thunderbolt3 connection -- and I believe even thew new Pro Display XDR connects with a single TB3 cable.