I think anonymity is heavily coupled with privacy, if someone knows my account is linked to my phone number, that’s a very strong form of fingerprinting. Even if E2E encryption is perfect, it takes one bad actor on the the reciever end of my message to both identify who I am through my phone number and leak my message. If just my message is leaked and there’s no fingerprint leading to me, I am still safe. Real example: It took Proton leaking the IP address of a climate activist to the state to get them arrested, not a hole in their E2E mail encryption. A phone number is potentially an even stronger identifier.
I think anonymity is heavily coupled with privacy, if someone knows my account is linked to my phone number, that’s a very strong form of fingerprinting. Even if E2E encryption is perfect, it takes one bad actor on the the reciever end of my message to both identify who I am through my phone number and leak my message. If just my message is leaked and there’s no fingerprint leading to me, I am still safe. Real example: It took Proton leaking the IP address of a climate activist to the state to get them arrested, not a hole in their E2E mail encryption. A phone number is potentially an even stronger identifier.