Only the latest version will receive security updates.
The OPC Foundation publishes security bulletins that affect software that it maintains or distributes. In many cases these bulletins will affect code that OPC vendors incorporate into their products. As a result, vendors will have to patch their products to address the vulnerabilities identified.
All the bulletins that have been published are available here.
Any vulnerabilities or security concerns should be reported to ‘securityteam AT opcfoundation DOT org’. A PGP key to encrypt any sensitive security report can be found here.
Complete information can be found here.