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I wonder if Apple dropped it because of some proprietary licensed IP, such as Intel WiDi, making the feature possible.

No, it was when the displays went to Retina. There was no way to drive them at 5K externally. Thunderbolt didn't have the bandwidth to do so.

They dropped it before the switch from Intel chips; my 2015 iMac could do it (I think) but my 2019 Intel iMac can't.