Neat, wonder why they stopped. Sounds like the Retina models might have too high of a resolution to go over a single Thunderbolt connection but it should still be able to do a scaled up resolution...
I don't have the answer to why they stopped supporting this, but I don't think it's because the resolution is too high for a single Thunderbolt connection. I have an LG 5k that works over a single thunderbolt3 connection -- and I believe even thew new Pro Display XDR connects with a single TB3 cable.