As I mentioned elswhere, I can confirm RX 480 works OOB, no Whatevergreen, no RadeonDeInit, nothing. 4 GB version. There is no reason RX 570 4 GB shouldn't work (curiexr).
UtterDisbelief: yes, it's un-Mac like. However, at the same time, it's completely Apple like. If you watch a history of Macs and have used them in past (I used a LOT of them between a generations, not bad for guy from Central Europe), you would know that adapters and converters and boxes are like tied to many of them. It's pretty usuall Apple changes connectors between generations, sometimes between a small updates of same model. Or completely removes interface on which many users rely (like FireWire, or Ethernet). If you saw a "miracle" display adapter for PowerMac 6100 you would know. Now they at least use standard connectors, albeit for price of removal of the others, like that missing Ethernet or multiple ports on Macbook, just for a sake being Hipsterish.
Sometime I have to wonder looking on these adapters and boxes, who in Apple has a huge stock reserve in these companies.