[greiskul]

You are mistaken about WhatsApp. It does end to end encryption by default. What needs to be explicitly enabled is that theoretically WhatsApp/some bad guy could steal an WhatsApp account, and it would create a new public key, and if you didn't turn on security notifications, you might not be aware that the person you think you are talking to changed. You also should verify that the end to end encryption keys match, but most users are not paranoid to that level.

[_trampeltier]

I heard a couple of times, after people get a new phone number, after installing WhatsApp, they see the conversation from the old owner of this phone number. How can this happen then?

[lieuwex]

If you really mean conversations of the original owner, I can't believe that's true after e2e has been enabled.

But it could be that messages sent to the original owner are received, since WhatsApp automatically re-encrypts and sends messages if the message has not been received yet and the key has been changed. So basically that would mean the message would be sent when the previous owner already changed their number. So the people shouldn't have sent the messages at all.

WhatsApp e2e encryption just makes sure that the only person that can read the message is the owner of the number, not necessarily the person you want to send it to.

[StavrosK]

WhatsApp also does backup.

[lieuwex]

In drive&icloud(?) AFAIK, so that would be linked to the owner of the phone at that time.

[CGamesPlay]

Doubtful that they are seeing the old conversations, but people with the old phone numbers can send new messages to the new account with this phone number, continuing an old conversation.