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Proxy Please: Help People Connect to Signal
(signal.org)
Technology for a Solar-Punk future.
Airships and hydroponic farms...
That's what they like you to think yes. But it is only the message content and not the various forms of metadata that is protected by their encryption scheme.
It's also the metadata. Profile and contact discovery is also private. I don't think they have anything except your IP.
Seal sender is a nice idea, but since you can easily run timing attacks on centralised infrastructure it is pointless for Signal, or rather you have to trust them that their infrastructure is not compromised.
They also store device ids for push notifications via Google/Apple.