In the time since the topic of redis dependency for multiple processes last came up it seems socket.io created an official mongodb adapter: https://github.com/socketio/socket.io-mongo-adapter
(Docs: https://socket.io/docs/v4/mongo-adapter/) - though it works only on replica sets (due to the use of change streams)
There is also a similarly recent Postgres adapter: https://github.com/socketio/socket.io-postgres-adapter (Docs: https://socket.io/docs/v4/postgres-adapter/)
So it should be now possible to remove the requirement for redis here.
