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any instance a user federates with gets a gigantic copy of all of their metadata,
No, it does not. Instances get metadata only for the chat rooms in which they participate, not all of a user’s metadata.
When chatting with someone on Matrix like you would with text messaging, only your instance and your contact’s instance are involved. Because they have to be, in order to exchange messages. Just like every other chat protocol that uses servers, including XMPP.
