Can ArcGIS 10.2 client direct connect to 10.6 server without problem? I have several customers who are currently on Arc GIS 10.2 client and server. But they want to move to 10.6 server but keep the client 10.2 (due to application compiled with 10.2 client). One customer has SQL server 2014 and another use SQL server 2016.
I know for ArcGIS 10.2 client to connect with 10.6 server that use Oracle 12, it requires a patch (ArcGIS 10.2.1 for (Desktop, Engine, Server) Support for Oracle 12.2 Patch). Is there similar thing for SQL Server?
A lot information about your case is documented
for ArcGIS Server here
and for the Geodatabase here