People have posted that it works from Nvidia RTX GPUs in Boot Camp (supports DSC so it only takes one USB-C or DisplayPort connection to allow 6K up to 12 bpc).
I think AMD 5300M, 5500M, 5700XT, W5700X, W5700 all support DSC. So they should allow 6K in Boot Camp (depending on AMD Windows drivers?).
I'm not sure Boot Camp allows 6K from GPUs that don't support Display Stream Compression (like the 580X, Vega II, and Vega II Duo) (the question is: do the Titan Ridge Thunderbolt controllers allow dual HBR3 connections in Windows like they do in macOS? - maybe, if the firmware is the same between OS's).