You mean Google is discontinuing the API? (They had an API? They removed XMPP federation and eventually support, if that's what you mean).
The libpurple/Hangups projects for chat clients is the work of reverse engineering the existing Hangouts protocol (which apparently heavily relies on protocol buffers).
The libpurple/Hangups projects for chat clients is the work of reverse engineering the existing Hangouts protocol (which apparently heavily relies on protocol buffers).