The plugin site for an ANT Mojo doesn't shows the 'since' information for goals and parameters.
The problem seems to be in the 'PluginMetadataParser.java'; this class gets the Mojo data object and convert it to a MojoDescriptor. Just two lines are enough to pass the 'since' data to the descriptor (and then, include it on the site).
I attach the patch and two tests cases for Maven 2.2.1 and Maven 3.0. I've test the patch with maven-plugin-plugin v2.5.1, v2.9 and 3.1.