WhatsApp has a cut-off date for a set of smartphones by the end of the year 2021. These devices will start to see their “end of service” message in WhatsApp starting on November 1, 2021. Before then, though, WhatsApp has a set of requirements that stand at Android OS 4.1 and newer, so long as that device is able to receive SMS or calls during the verification process. WhatsApp does not support Wi-Fi only devices.
Starting in November of 2021, WhatsApp will change its base requirement for Android to Android OS 4.0.4 or older. You may already have a notification in your WhatsApp app if you have an older Android device right now. If you are able to update your Android device to run Android 4.1 or higher, you’ll very likely be able to continue using WhatsApp through the future.
If you are using an iPhone, your options are slightly more limited than they might be on Android. You will need to have an un-modified version of the iPhone’s operating system in order to use WhatsApp. They do not “explicitly restrict” using jailbreaking or unlocking devices, but the mods themselves may affect your device’s functionality, in turn stopping WhatsApp.
The WhatsApp requirement for iPhones running iOS at the moment is iOS 10 or higher. There is no current cut-off date for legacy iPhone support IF you make sure your operating system has the latest update available from Apple.
If you have an iPhone 5 or newer, you’ll likely have access to iOS 10 or higher and WhatsApp for the near future. If you own a device older than the first iPhone 5, you’re pretty much out of luck.
The only other option for mobile devices is KaiOS. WhatsApp works on KaiOS 2.5.1 or newer. This means you’re going to need a JoiPhone or JoiPhone 2 – no other phone is explicitly supported with KaiOS as yet, especially if it’s running a KaiOS version older than 2.5.1.