There is a utility class that the built-in plugins may use to read the proxy information from the Forge user configuration. This is the org.jboss.forge.shell.util.ProxySettgins class, which is part of the forge-shell project.
However, the plugins that are not part of that module do not have access at runtime to this utility class. So if they need information about the HTTP proxy, the plugin writers have to know all the details how these are set in the ~/.forge/config.xml file.
The solution is to move the ProxySettings class to the same package, but in the forge-shell-api project. The latter can be safely accessed by the plugins.