A knowledge base article about What Operating Systems Work with Duo Mobile? provided by the UC Berkeley IT Service Hub - Knowledge Portal
Android: The current version of Duo Mobile supports Android 11.0 and greater, as well as Android Go 8.0 and greater. Users on Android versions older than Android 11.0 will not be able to download the latest version of Duo Mobile from the Play Store. This removal from the Play Store will not affect mobile app authentications for users who have already downloaded the app.
iOS: The current version of Duo Mobile supports iOS 15.0 and greater, but we cannot guarantee Duo Mobile's functionality on preview/beta software provided by Apple. Users running versions older than iOS 15 will not be able to download the lates version of Duo Mobile from the App Store. However, users running versions of iOS older than 15 who previously downloaded Duo Mobile with their existing Apple ID will be able to download the prior version compatible with their device.