Signal (and many other messengers) stores push tokens in their database in plain text. Which allows law enforcement to subpoena them (though signal has never publicly admitted this happening) and then law enforcement uses these push tokens to subpoena apple to get personal information associated with it like your full name, address, phone number etc.
This way Law Enforcement can correlate your personal information with your signal account which means participants in conversations are deanonimzed. (Though content is still encrypted).
If you organize in large signal groups (or any Messaging app using push notifications) there is a high chance law enforcement and intelligence agencies can tell that you're part of a group or not.
I don't understand why nobody is talking about this more. It's extremely problematic.
Apple started publishing stats on how often this happens now. They were gag ordered to not speak about it before but they have changed course (https://www.reuters.com/technology/cybersecurity/governments-spying-apple-google-users-through-push-notifications-us-senator-2023-12-06/)
You can find the stats here:
https://www.apple.com/legal/transparency/push-token.html