Decentralized encrypted group messaging has been called impossible by the people who built the best encrypted messengers on earth. Moxie Marlinspike designed Signal as deliberately centralized because he concluded that federated protocols freeze in time while the world moves forward, and even the IETF's multi-year effort to build MLS still required a central delivery service.
Now the Marmot protocol offers the first credible specification for solving this problem at the protocol layer, and interoperable implementations are already shipping.