> Law enforcement asks signal if they have an account for a phone number, signal saying "yes, here's when they created it".

Law enforcement says that the suspect chatted with some username/told people to contact him by his Signal username, then they go to Signal and request the linked phone number, which is then linked to the ID shown when the card was bought.

This only works as long as the username is active/unchanged. It would probably be better if usernames were never linkable to phone numbers, but if your threat model requires a persistent, non-ephemeral username to remain anonymous when targeted by law enforcement that has access to your telecom records and warrants... that's going to require a pretty high level of opsec.

The UX on usernames in Signal might be non-ideal. It might be helpful to have a toggle that regularly cycles your username if that's important for your threat model.

"Get me all the numbers which talked to X, including all the numbers".

You won't get the actual plaintext messages, but the contact graph + metadata (timestamps) are pretty sensitive.

Signal doesn't store the graph, nor does it log message timestamps.

How can you know without access to their servers?