Apple dropped "Target Display Mode" in 2014 with the 5K iMac — and it never came back.
TargetBridge brings it back via software, streaming your screen to the iMac at up to 5K over a direct Thunderbolt cable.
Since v2.0.0, you can now even connect multiple iMacs to get multiple gorgeous 5K displays!
FOSS, under the MIT license.
Disclaimer: I am a project contributor, but not the original author of this project.