They have published requests from the law enforcement and their responses to these requests. The only unencrypted data they have is the phone number, a date of sign up and a date of the last login. That is it, everything else is encrypted and they cannot access it whatsoever.
I can go to the corner shop/local garage right now, buy a SIM card for 99p and then buy a top-up voucher in cash to have a completely anonymous phone number.
Albeit is the UK in Europe again? 🙈
edit: where I would be worried if my privacy was on the line is I could also go to the local pawn shop / Cash Converters to ensure that SIM card isn't associated with an IMEI I've previously used and buy in cash a cheapo phone.
Same in a few other European countries. I'm doing it right now with a few SIMs. You can also go on holiday to another country, get a few temporary SIMs there for a few quid and fly back home with the "contraband". Really not hard.
I don't know if you're in the UK right now but I can tell you right now that I can go round the corner and buy a SIM card in cash plus a top-up voucher, from someone like this guy
So what? The law enforcement knows you have an account and knows the sign up date and last login. That doesn’t affect your privacy whatsoever. Besides, Europe isn’t a monolith. You can absolutely buy and use a SIM card without disclosing your name in some countries.
You also don't need to show any ID for a business to meet "know-your-customer" regulations. Can you get a phone number without revealing your identity?
Sometimes, but that's it. Authorities and signal itself can only say "this number has account with us since $signupDate and used it last on $usageDate". Signal can't say "we know $number is talking to $otherNumber" nor can they say "$number is in $group talking to $users".