MBP 2016 + iMac 21 (mid 2011) using a thunderbolt 2->HDMI cable and an HDMI->type C adapter? I was wondering if that would work? I've found out this to be the cheapest option for me, albeit an awkward one.
Both of these adaptors are design to output to a HDMI TV or monitor. The idea of going backwards and have the HDMI converted as input to a Mac is invalid.

Also, there is more going here than just using the Mac as a display. Each Mac gets access to the to the drives on the other Mac. You can also share the screen on the Mac being used as a display. For example, below is a image of what is was shown on the display of a 2013 iMac. This Mac was in Target Display mode. In other words, the 2013 Mac was being used as a second monitor connected to a another Mac. In the image, you can see the other Mac was using the Screen Sharing application. The Screen Sharing application window is showing the desktop of the 2013 iMac.



P.S. Before you run out an buy the correct adaptors and cables, you might want to read my answer to the question Can I use an iMac 2014 as a display for a Mac mini.