WhatsApp recently introduced a disappearing messages feature that gives users the option of having their messages automatically disappear after seven days. Users can turn on the feature for each chat individually. Our guide will show you how this works.
Note: These screenshots were captured in the WhatsApp application on iOS. Also note, only group administrators can turn disappearing messages on or off for group chats.
Step 1: On the Chats screen, tap the chat you wish to edit.

Step 2: Tap the contact’s name (or the group name) at the top of the screen.

Step 3: Tap “Disappearing Messages.”

Step 4: Tap “On” to turn on disappearing messages for that chat.

https://www.adweek.com/media/whatsapp-how-to-turn-on-disappearing-messages/