The latest major version, SME Server 10, is still actively maintained, though there has been some delay in releases due to the limited number of developers.
This is not true.
The core is based on CentOS 7 which we do not maintain and is EOL.
A number of companies offer paid support/repos for this.
We no longer maintain the v10 SME code as we are busy on v11.
There will be no more releases of v10.
However if we discovered a security issue with our code we would release a patch in extremis.