Is it possible to use a Mid 2011 iMac as a second monitor for a 2018 MacBook Pro? I've been unable to get the Target Display mode to work on the iMac when the two are connected via a Thunderbolt 3 (USB-C) to Thunderbolt 2 adapter and a thunderbolt cable.
With a qualified iMac yes it should work, I was told this by Apple support. However there is a problem with the 2018 MBP and it does not. There is a large thread on this in the Apple forums:
Someone there recently figured out a workaround by daisychaining their iMac but we shouldn't have to buy more expensive devices to get it to work. Please report your issue to Apple support and make sure they escalate it to an engineer. Only by everyone reporting do we have a chance of getting this fixed.
I have the same problem. I use my 2011 iMac (running on High Sierra OS) for target display to MacBook Pro (Retina, 15-inch, Mid 2014). The iMac was running very smooth as my target display. A few days ago, I just got 2018 MacBook Pro (running on High Sierra OS version 10.13.6 (17G2307)) and it's no longer able to connect to the 2011 iMac. After I read the discussion here, I chat with the Apple support. He told me to use Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter. I will get this cable tomorrow and will give you an update.