WhatsApp hasn't officially announced it yet, so we can't say when the feature will be available to all users.
The new feature has started rolling out, and should be available with the latest iOS and Android WhatsApp versions, according to WABetaInfo. The feature is quite self-explanatory but here's how it will work. When you search for an unknown number on WhatsApp, you're directed to save the number to start a new chat with them. This might not be the requirement all the time, and one that you would like to avoid.
With the latest update you'll be able to chat with numbers not saved in your phone. You can do this by following these steps:
WhatsApp hasn't officially announced yet so we can't say when the feature will be available to all users. For now, you can update WhatsApp to the latest version from the App Store or Google Play Store. Then you can try out the steps mentioned above to see if you have received the feature. Otherwise, it will most likely be a phased rollout so it should be available on your phone soon.