Signal uses phone numbers as identifiers. Due to KYC laws phone numbers are closely bound to real-world identities.
If the police is able to unlock one person's phone, even if that person used self-deleting messages, they immediately know the identity of every other person that person chatted with.
Signal might as well have used your SSN as your account ID, that would have been just as private.
Luckily Signal has finally started fixing that in the past few weeks.
