you have legitimate concerns, but you’re very wrong about this. zero-knowledge proofs are not only the technology behind donation badges and backup payments, but also behind groups. they can’t tie you to a specific donation just like they can’t tie you to a specific group.
the great thing about Signal is the client is designed to already not trust the server. in fact you can show that Signal is perfectly safe without even looking at server code.
ZKP is also used for verifying things like username character set and length without actually revealing contents which is super cool and comprises of math waaaayyyyy above my head.
