2017 4K, 5K iMacs won't support Target Display Mode, despite Thunderbolt 3
Apple has definitively stated that new 2017 iMacs will not support Target Display Mode, a feature that previously allowed some iMac models between 2009 and 2014 to use their built-in display as an external monitor for another Mac connected via Mini Display Port or Thunderbolt.
First supported on the Late 2009 27 inch iMac, the feature provided a convenient way to turn an iMac's internal display into an external monitor, and ostensibly served as a way to prolong the life of the machine after the computer itself grew obsolete. However, if there is a real component failure that stops the Mac from working, Target Display Mode would also fail.
The feature was continued in the Mid 2010 refresh of the same model, then extended across the iMac lineup between releases in Mid 2011 and Mid 2014. However, the feature was not supported in the Late 2014 5K iMac, apparently due to the fact that a single Thunderbolt 2 cable could not drive the full resolution of the new ultra high resolution display. No iMacs since have supported Target Display Mode.
With the recent release of new 4K and 5K models supporting Thunderbolt 3, which does have the bandwidth to drive a 5K (5120x2880) resolution, 10 bit display with 1 billion colors, the obvious question was whether Apple would revive the feature.
However, an Apple spokesperson told AppleInsider that 2017 iMacs are not designed to support Target Display Mode, and clarified that the feature is not planned to ever reappear. An Apple support page describing the feature, dated March 2017, also states that "iMac (Retina 5K, 27-inch, Late 2014) and later iMac models can't be used as Target Display Mode displays."
There are other ways to emulate the hardware trick in order to stream the display of one computer's desktop on another machine, but Apple's hardware based approach offered a true monitor-quality use of the internal display.