Here’s my take on this.
Recovering a message that’s already been deleted on an iPhone presents a few challenges. The simplest, though least reliable, method is checking the Notification Center. If the notification hasn’t been cleared, you might still see the preview there.
For a more thorough approach, you could consider restoring the phone from an iCloud or iTunes backup. The major downside is that the backup must have been made before the message was deleted, and you would lose all new data created since that backup.
Looking forward, a monitoring tool is the most direct solution for this issue. These apps are designed to log all incoming messages as they arrive, creating a record that exists independently of the WhatsApp app itself. This way, even if a message is deleted moments later, you’ll have a copy of it. It’s a classic trade-off between reactive and proactive solutions.