There was some pressure to get SecondLife open-sourced and federated, but IIRC it didn't really get anywhere.
On the other hand, projects like Croquet/Qwak/Cobalt have been open-source for over a decade; are federated/P2P; already have "hyperlink portals" like parent commented; support existing standards like XMPP, VNC, etc.; have been ported to Javascript; etc. And yet, they seem effectively dead :(
The SL client ("viewer") has been open source for a very long time, and there used to be a thriving ecosystem of third-party forks. Nowadays Firestorm [0] is the one everyone uses.
The server software remains closed source. OpenSim [1], a community-driven reimplementation with federation support, has been around for a while but as you say hasn't really gone anywhere since the original wave of metaverse hype died down.
On the other hand, projects like Croquet/Qwak/Cobalt have been open-source for over a decade; are federated/P2P; already have "hyperlink portals" like parent commented; support existing standards like XMPP, VNC, etc.; have been ported to Javascript; etc. And yet, they seem effectively dead :(