I just skimmed through the specifications of two decentralized protocols and I use applications that have implemented them.
Matrix (M) and ActivityPub (AP) are the 2 protocols I am talking about. Both of them are very similar in terms of what they have implemented.
1. Client-Server API
2. Server-Server API
Both protocols uses JSON to exchange info. Both of them run on top of HTTP(S). M includes WebRTC for VOIP calls, which can also be plugged into AP as well.