Matrix is a standard for a federated messaging system. Users can choose from a variety of homeservers to sign up on and are able to communicate with users of other homeservers.
Bridges are available for IRC, Discord, Gitter, and other platforms.
Homeservers running Synapse can be configured to only keep message data for a specified amount of time.
- The Element client has the "Export chat" button allowing archiving one channel at a time.