What would stop them from subpoenaing all information from your personal server?

If you’re a drug dealer and the FBI sends you a subpoena—you could simply….not respond.

There’s no personal information tied to your account.

There is actually a bunch of metadata tied to your account and your room. That’s partly how they caught that kid with the Pentagon leaks.

And again, there may be other services between the clients and the matrix server that collect personal data (e.g. reverse proxies, load balancers).

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If you are someone who ostensibly cares about privacy and security (like a drug dealer) why would you rely on the benevolence and security hygiene of a stranger you can’t audit? Instead of using a known good actor, like Signal or SimpleX, or no actor, like Briar.