When 5K iMacs first came out in 2014, they were mostly limited to displayport 1.2, which was unable to support 5K without either dropping to 45Hz or using chroma subsampling. So the 2014 version is excused, and we can excuse the 2015 version too.

But with the 2017 revision, they had a displayport 1.4 GPU in there, and there was no good reason for leaving display target mode out.

And they've left it out ever since.

Streaming compressed video with a significant latency penalty doesn't count. But if it we act like it does count, then they lose their excuse for removing target display in the first place, because even in 2014 it was possible to do 4:2:0 5K @ 60Hz over a normal connector. That looks better than compressed video does!

Yes the previous iMacs sent a signal. Their current computers don’t receive the signal. This is not about the 5K display, you’re missing the core problem. It stopped working when the silicon line came out. You can’t get any iMac to work in target display mode on silicon Macs.

That’s not a technical limitation, that’s laziness. Go read more about this. It simply doesn’t work anymore without 3rd party solutions.