What are the Apple-provided drivers for the AMD Pro Vega 16/20 for Windows 10?
The answer is Yes, they are supported in Windows. The Windows Support image which gets downloaded from Boot Camp Assistant.app if you are on a 2018/2019 MBP shows AMD drivers. These can be downloaded manually using the brigadier tool, by specifying the correct model name (e.g. MacBookPro15,3).