WhatsApp might forward all message metadata to ads manager so that the user's ads profile can be further improved. If the ads manager platform then goes down, the pipes for this data flow will fill up, and if it is not built to be able to handle a backlog WhatsApp may go down, too.
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.
That specific classname didn't change since the beginning of the project, I had a breaking change on WhatsApp's side one time (was several months ago already).
I'm aware of https://github.com/tulir/mautrix-whatsapp which I'm personally interested in, which uses a reverse engineered implementation of the WhatsApp web protocol.
I think "kak" still would be translated better as "shit", you wouldn't curse with "POO!". But in Dutch you can curse with "KAK!", just as 'SH*T!' in English.