Xtreix
When a message recipient flags a WhatsApp message for review, that message is batched with the four most recent prior messages in that thread and then sent on to WhatsApp's review system as attachments to a ticket.
Although nothing indicates that Facebook currently collects user messages without manual intervention by the recipient, it's worth pointing out that there is no technical reason it could not do so. The security of "end-to-end" encryption depends on the endpoints themselves—and in the case of a mobile messaging application, that includes the application and its users.
An "end-to-end" encrypted messaging platform could choose to, for example, perform automated AI-based content scanning of all messages on a device, then forward automatically flagged messages to the platform's cloud for further action. Ultimately, privacy-focused users must rely on policies and platform trust as heavily as they do on technological bullet points
So far, I haven't seen evidence that this is happening automatically without users manually flagging messages. So as long as you trust your message recipients, it shouldn't be too concerning for lower threat model users. For large whatsapp groups, this could be concerning.
I feel like some weight should be given to the SEC fining wall street firms billions of dollars for allowing employees to use WhatsApp during COVID, due to whatsapp not preserving communications, which goes against federal recordkeeping requirements. If WhatsApp maintained records, I feel like that may have come up during these various legal proceedings.
But yes, it's totally not worth the risk to use WhatsApp for higher threat model communications or if you're being targeted, especially due to the message metadata leaks. But I personally think WhatsApp still fits in well for most people to casually message friends and family due to its combination of usability features, mostly acceptable encryption promises, and by the sheer fact that people are more likely to use it.