As you observe that someone deleted message in your WhatsApp conversation then you see message “This message was deleted”.
When someone send message to you then this app store notification into your device, as a result you can view deleted message.
Now lot of mobile apps available on play store saves all notification of all mobile apps.
Now a days, notisave mobile app is buzzing all around the world.
Download app and end your curiosity regarding deleted message. Here is URL of NotiSave on android play store.