The new functionality is available in the most recent WhatsApp beta for Android, version 2.22.25.20. Once you send such a message, the receiver is unable to share or forward it.
It should be noted that WhatsApp already offers this capability for photographs and videos; it is now being extended to text messages. When you send a view-once picture or video, the recipient cannot capture a screenshot of it; presumably, the same capability will also be available for view-once texts.
