The missing part is the action part. An email (or phone number, or address) is an identifier. Asking whether this identifier is public or private misses the im

The missing part is the action part. An email (or phone number, or address) is an identifier. Asking whether this identifier is public or private misses the important thing, which is the action that can be paird with the identifier. So therefore, there's no universal answer to whether the identifier should be public or private. It's a case by case basis, when paired with an action. For example, i don't want a shop to see me buying condoms, so shops shouldn't get my email address (or phone number).