Ok, I installed Windows Server 2019 Standard in a vmWare machine on my Windows 10 laptop and both A-Shell and ATE appear to be running 6.5.1684.6 on it just fine. The only problem which I ran into was that ATSDSRV.EXE did not install the ATSD service. Not yet sure what the problem is there. But you can always set it up to run as an app (which I did) and it appears to accept connections without any problem.
I didn't do a huge amount of testing, so caveat emptor.