First spotted by WhatsApp update tracker WABetaInfo, this feature allows users to send media that disappears after the receiver has seen it. Unlike the disappearing messages feature, View Once doesn't have a time or expiry when the sent media automatically disappears, until the receiver has viewed it once.
View Once media won't preview or appear in the notification panel or on the lock screen. This feature works for both individual chats and groups. The sender can see whoever viewed the media in the group info, and will as well be notified when a Wha recipient opens the sent media during one-on-one chat.
However, if you have read receipts turned off, you won't be able to know if and when the recipient has seen it.
For you to be able to use the View Once feature, you have to be on WhatsApp beta version 2.21.14.3 for Android. Funny enough, recipients don't have to be on WhatsApp beta before they will be able to receive View Once media.
Another thing here is that is WhatsApp doesn't protect against screenshots or recordings, nor will the sender get an alert that the View Once media was captured. This flaw to me defeats the whole purpose of the View Once feature.
While WhatsApp beta for iOS waits for the View Once feature, there is no mention on when it will be rolled out to a stable version of WhatsApp for Android.