I operate an E2EE messaging service that uses email instead of phone numbers as user IDs

My servers also don't keep any record of conversations or "buddy lists". Yes, contact lists get wiped when changing device and email addresses aren't as easily stored in phone books (I don't even access a user's phone book for that matter). Just saying that it's possible to not use phone numbers. Granted not as convenient once you've built your entire user base off of their phone numbers

Email is more private though