OPC Foundation has discontinued the development of the UA SDK and it is now available as a sample project only for members.
It is recommended to check the commercial SDKs, which will provide you a real product level support, etc. The situation is now similar with all major environments: .NET, Java and C/C++.
There are already several SDKs which have been developed after OPC Foundation made the decision to leave the field to the SDK vendors.
OPC Foundation is now concentrating on the maintenance and development of the OPC UA Stack components only.
The stack components take care of the actual communication protocol, whereas the SDKs provide implementations for the basic services. Take a look at https://www.prosysopc.com/opc-ua-about.php for a detailed explanation.