The Target Display article (https://support.apple.com/en-us/HT204592) indicates my iMac can connect to another Mac via a Thunderbolt cable. A MacBook 2017 has (only) a USB-C port, not Thunderbolt. Is there adapter that will allow this to work?
iMac 21.5", macOS 10.13
I don't think so. The MacBook port is not "Thunderbolt 3 (USB-C)", just USB-C. It looks like the same kind of port, but apparently doesn't have the Thunderbolt architecture. So, any solution would have to somehow translate Thunderbolt <---> USB-C.
You are right. That adapter requires a Thunderbolt-3 port, and won't work with USB-C only ports. There is no way to run a Thunderbolt display (or Thunderbolt iMac in Target Display Mode) from a non-Thunderbolt computer.
You could run screen sharing software on the iMac so see the MacBook screen, but it will be slow.
iMac mid-2011 as Target Display for MacBook 2017
