Out of the box, Maven 3 only supports file:, http: and https: as transport protocols. Maven 2 adds scp: to this list.
If you try to deploy a site with an unsupported protocol, you'll get an error:
[ERROR]
Unsupported protocol: '...' for site deployment to distributionManagement.site.url=...
Currently supported protocols are: ...
Protocols may be added through wagon providers.
For more information, see http://maven.apache.org/plugins/maven-site-plugin/examples/adding-deploy-protocol.html
As explained in the error message, to deploy a site with a non-built-in protocol, you need to add the corresponding wagon provider.
You can add it either as a global extension, or declare it at as a dependency of the site plugin:
<project>
...
<build>
<pluginManagement>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-site-plugin</artifactId>
<version>3.2</version>
<dependencies>
<dependency><!-- add support for ssh/scp -->
<groupId>org.apache.maven.wagon</groupId>
<artifactId>wagon-ssh</artifactId>
<version>1.0</version>
</dependency>
</dependencies>
</plugin>
</plugins>
</pluginManagement>
</build>
...
</project>