Mike,
When Antelope Elk performed the upgrade to sWOIS in March of 2019 we were upgraded to Windows Server 2012 because that was the currently tested version that was available.
We did have in our purchase order (and it was agreed to in advance) that Wartsila would supply the platform on the newest release of windows server available. Because of time lines, Wartsila decided to perform the install on 2012 and then come back later and upgrade the servers to 2016 at their cost after it had been tested by the factory. We expect that upgrade to happen in April of this year.