Will the 2018 Mac mini be able to use my 2011 iMac in Target Display Mode?
Solution 1:
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.
Solution 2:
I think I have a definite answer to my question, so I'm putting this up for anyone else in a similar situation.
I bought a 2018 Mac mini, and set it up today and I can confirm that Target Display Mode works using a non-Apple USB-C to mini DisplayPort adaptor, plugged into the mini's Thunderbolt3 (USB-C) port. Target display is an iMac mid 2010 27". I also bought an Apple Thunderbolt3 to Thunderbolt2 adaptor and tried using it for Target Display Mode, but it doesn't work.
The Mac mini setup went pretty well actually, so I'm pretty happy about that.