This restriction feature is called Send Messages. Users can find it in Group Info under Group Settings. Keep in mind, these options will only be visible if the user is an admin member in a WhatsApp group.
If Send Message is set in the Admin Only option, group members will receive a notification above the mailbox. The message composes box will disappear.
Only this new feature can be enjoyed by the user WhatsApp on the IOS platform. But certainly will also be present on Android in the near future.
iOS users are asked to update to version 2.18.70 to taste this new feature. While on Android later in version 2.18.201.