rMW5c8245acdffd8ff5f52638b85520c54b0aed6b9b introduced a method to setup proxy respecting both $wgHTTPProxy configuration variable and http_proxy environment variable.
This commit for example fixed features like InstantCommons behind a proxy.
But some extensions also rely on proxy configuration, and also have to check both the proxy sources, as they don't rely on Http class. It would be convenient to offer a static method returning the proxy value.