A clock icon should then appear next to affected messages, signalling that they will not last forever.

WhatsApp Disappearing Messages Clock

Though we have been unable to replicate its screenshots in the beta version of the app, it claims the changes are visible in version 2.20.83/2.20.84, so it might be dependent on region or available to select users only.

Should the feature be available to you, you'll be able to access disappearing messages by tapping on the recipient or group name at the top of a conversation, and scrolling down to Disappearing Messages, which should appear between Media Visbility and Encryption.

The move beefs up WhatsApp's exisiting ability to delete sent messages, whereby you are able to delete messages provided they have not already been read by the recipient.