WhatsApp has a deadline for a set of smartphones by the end of 2021. These devices will start seeing their “end of service” message in WhatsApp from November 1, 2021. Prior to that, however, WhatsApp has a set of requirements that apply to Android OS 4.1 and newer, as long as this device is capable of receiving text messages or calls during the verification process. WhatsApp does not support Wi-Fi only devices.
From November 2021, WhatsApp will change its basic requirements for Android to Android OS 4.0.4 or older. You might already have a notification in your WhatsApp app if you currently have an older Android device. If you are able to update your Android device to run Android 4.1 or higher, you will most likely be able to continue using WhatsApp in the future.
If you’re using an iPhone, your options are slightly more limited than they might be on Android. You will need an unmodified version of the iPhone operating system in order to use WhatsApp. They don’t “explicitly restrict” the use of jailbreaking or unlocking devices, but the mods themselves can affect the functionality of your device, in turn shutting down WhatsApp.
The WhatsApp requirement for iPhones running iOS at the moment is iOS 10 or higher. There is no current deadline for old 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 will likely have access to iOS 10 or higher and WhatsApp in the near future. If you have a device older than the first iPhone 5, you’re out of luck.
The only other option for mobile devices is KaiOS. WhatsApp works on KaiOS 2.5.1 or newer. This means that you will need a JoiPhone or JoiPhone 2 – no other phone is explicitly supported with KaiOS yet, especially if it is running a version of KaiOS earlier than 2.5.1.