We had a development system with SQL 2016 which appeared to work fine. We have reverted back to SQL 2012 for support on our production system though.
The reason why SolarWinds haven't "validated" SQL 2016 is because they will drop support for 2012.
Many users and companies still use 2012 as a solid solution so it wouldn't be good if they dropped it so suddenly, though I do agree that 2 years is really enough time to start moving on this appropriately.
I can confirm that SQL 2016 does work from versions 12.1 onwards. I cannot confirm whether you will have issues down the line and would recommend that even though you can proceed with upgrading, you should delay it until it becomes supported.