Sent WhatsApp messages can be deleted, but they don't really go away
If an Android device has already been compromised by malware, the notification log could be a way for anyone to read WhatsApp messages, deleted or not, and that's a huge security problem for an app that prides itself on being private and secure.
If a user deletes a WhatsApp message they may assume the recipient won't get it, and they very well may not if they don't know how to access the notification log.
Android blog Android Jefe found out that WhatsApp's message delete feature is leaving notifications behind that can be read in the notification log.
During message transmission that may be the case, but sensitive notifications stored in a log make this an issue of endpoint security.