As per the Wabetainfo report, WhatsApp will soon bring Disappearing Mode in the privacy section. As of now this feature will be available in the future but the testing is ongoing. You can enable it within Contact Info and Group Info, but it can happen that you have to manually enable it when a contact starts a new chat with you: for this reason, WhatsApp is now developing a new option in WhatsApp Settings > Privacy that will automatically turn on disappearing messages in new chats!
Wabetainfo has shared the screenshot of this upcoming feature which you can find below, when the user enables Disappearing Mode, new chats will begin with disappearing messages on. WhatsApp rarely adds new privacy settings, and we hope to see more in this section the next weeks. Stay tuned for more announcements about new features!