Apple's support page for the Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter lists support for Thunderbolt Displays and no support for Apple DisplayPort displays. However it doesn't say anything about iMacs with Thunderbolt in Target Mode mode. Does Apple's Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter support Target Display Mode on an iMac?
My guess is that it does since Apple's support page for Target Display Mode says that you have to use a Thunderbolt source for an iMac with Thunderbolt in order to use Target Display Mode.
Best Answer
Yes, I'm successfully running a setup with a 2017 13" MacBook Pro and a 2011 iMac. The original Apple USB-C to Thunderbolt 2 Adapter works fine for Target Display Mode.