https://github.com/waku-org/nwaku used by
xmtp is presently not decentralized but claims to be pursuing this.
it looks like messages might be stored off-chain (haven’t verified) in a separate p2p network, but there is a big software ecosystem for users to patch things on to.
a little reminiscent of early signal, where everyone was walled into this stupid phone app but it used a great protocol under the hood that could be facilitated to form secure communications with easy onboarding