[spoondan]

No. WhatsApp does not keep delivered messages, and, by default, messages are end-to-end encrypted and cannot be read by WhatsApp.[1] As shown in the screenshots of the reporting feature in the article, reporting causes your phone to send (forward) the relevant messages from your phone to WhatsApp as part of the report. For reporting a user or business, the relevant messages are "most recent messages" from the user/business. For reporting a group, the relevant messages are "most recent messages" in the group.

This feature does not change the privacy story for WhatsApp in any way. You already trust users you are chatting with to not copy or screenshot private messages and share them. You already trust the app to not forward or store your messages without your consent.

[1] See bullet point "Your Messages" under section "Information We Collect" on https://www.whatsapp.com/legal/privacy-policy.