The Signal servers essentially pass around encrypted blobs from one device to another. Most of the development work is on extending the clients to pass around different types of messages, so there's not much work that needs to be done on the server.
9 months is a long time to go without updates though, especially if you just got a ton of extra users. While I definitely see your point, GP also has a point about development clearly not being open. It's "code available" software, not "open source server" software.
