I am a firm believer that there are many privacy techniques you should focus on before encrypted messaging because they will offer you much more “bang for your buck,” things like good passwords, two-factor authentication, and even encrypted email. That said, I still believe that encrypted messaging is a critical part of a well-rounded privacy and security strategy. While the vast majority of our day-to-day conversations may be benign, it can still offer a lot of insight into who we are as people – our routines, likes, and personal thoughts. This information – mundane or not – is worth protecting.
Matrix is the worst option when it comes to avoiding metadata. A group chat with users on 10 different servers will create ten different places to store the metadata with no way for any user to delete or edit this metadata. Its a privacy nightmare.
How does xmpp handle it?
All decentralized protocols have this issue. The servers need to handle metadata for chat groups, like who is part of which group. If the servers are under individual control, nobody can force them to delete this data. The question is, do you trust a non profit organisation like signal to minimize and delete metadata (which court orders have proven they do) or do you trust all individuals of a group chat to do the same when you manually ask them to.