I'm building Talo (https://trytalo.com) - an open source backend for Godot and Unity games with leaderboards, analytics and authentication.
I've been working on making it easy to drop in socket-based multiplayer with "channels". Players can join channels and they can share messages, state updates or notifications over a socket connection. You can use it for chat rooms, lobbies/matchmaking or async multiplayer.
One recent addition is "channel storage": a shared space for players to read/write/update/delete data. This opens up saving and loading shared worlds between players in just a few lines of code.
Everything is open source, including the frontend dashboard, backend, Godot plugin and Unity package. GitHub here: https://github.com/TaloDev.