There are a large number of them, but most are sadly either abandonware or vaporware. The closest thing I feel we have right now is really matrix.org. User iden

There are a large number of them, but most are sadly either abandonware or vaporware. The closest thing I feel we have right now is really matrix.org. User identities are federated unfortunately, but this makes things that are going to be a problem on any fully decentralized system, like mobile push notifications, simpler.