That hasn't been the case for Signal for some months: https://signal.org/blog/phone-number-privacy-usernames/
You still require a phone number for sign up for Signal, but your phone number isn't visible to anyone you chat with.
That's irrelevant - the phone number is known to Signal and can be request by law enforcement. And, since it's been made pretty much impossible to buy a SIM in the EU without showing identification [0], this will allow law enforcement to link the account to you.
[0] IIRC the Netherlands is the only country left where you can buy SIMs without ID.
As far as I know, in Romania you can still buy and activate a prepaid SIM card without having to show your ID. There was an attempt a few years ago to make it mandatory to tie the phone number to an ID, but it was overruled by the Constitutional Court.
So how does this work? Law enforcement asks signal if they have an account for a phone number, signal saying "yes, here's when they created it".
Then what?
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.
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.
You won't get the actual plaintext messages, but the contact graph + metadata (timestamps) are pretty sensitive.
Maybe I'm missing something here, but if usernames are treated as ephemeral, what's the threat model here?
Wrong. Because:
> You still require a phone number for sign up for Signal
So, they have your phone number. What is displayed is irrelevant.
If they have your phone number (which they do), they will have to disclose it for any subpoena/NSL, so they do.