Will the 2018 Mac mini be able to use my 2011 iMac in Target Display Mode?
I think I found the answer, buried deep in Apples support doc for the Thunderbolt 3 to Thunderbolt 2 adaptor. 


  This adapter is not compatible with displays that use DisplayPort or
  Mini DisplayPort, such as the Apple LED Cinema Display. Thunderbolt
  and Thunderbolt 2 are not the same as Mini DisplayPort , though the
  ports have the same shape.


Apparently Thunderbolt 3 does not pass mini display port signals, so target display mode will not work if you have an older iMac or monitor that uses mini display port.

This makes a big difference to me, as I will have to buy a new monitor if I go ahead with a mac mini. At least thats what it looks like from the doc. I'd welcome anyone who has actually done this.