device_display_name is optional in the federation response. Current Synapse omits it by default, but it can still be sent when explicitly enabled. Older Synapse versions also exposed it through some federation paths.

I believe you are saying that my statement, “Federation copies all of that,” is incorrect. I agree. I should have said, “Federation copies most of that.”

However, the main concern remains: a homeserver can still collect the client type and version, the device being used, room names, topics, avatars, and other metadata. Much of this information is not necessary for delivering end-to-end encrypted messages, and its collection is concerning in itself.

Is there solid proof that it is used globally? Have technology experts conducted any research on it, or is verification simply impossible because they cannot inspect the code?

Fair point. My mistake.

Fair point. My mistake.

It's already a problem that it sends the client's version, device details, avatar, and bio at all. Those device details are transmitted during federation along with everything else.

https://spec.matrix.org/latest/server-server-api

Signal is free from this disaster, and client version and device details are only visible to the main server — not to mention the avatar, name, and many more items listed above being in plain text.

Don't they always?

How is that a bad thing