Second Life on GitHub
It's been open source since 2007. Linden Lab recently moved the sources from Bitbucket to Github.The Firestorm viewer sources are at "https://vcs.firestormviewer.org/". It's a major fork.Technical documentation of the client-server protocol is at "https://wiki.secondlife.com/"
It's out of date, but it helps.The real progress is that the build procedure has been cleaned up. It used to require ancient versions of Visual Studio on Windows. Firestorm can be built on Linux. I've built it and submitted patches that went in.This is just the client side. The server is proprietary. But there's a compatible server, Open Simulator. It's not a clone; it's in C#, while the original is in C++.There are meetings for third party viewer developers. Both inside Second Life, and on OSGrid, which is a third party grid of simulators. That really is the metaverse - a federated system of 3D worlds. It's sluggish, because most of the people running regions don't have enough server power. Think Mastodon for 3D.I'm been writing a new client in safe Rust for the last two years. I'm trying to see how much can be done in parallel. The viewer isn't ready for public use yet. I've posted videos made with it.