WhatsApp will primarily end support for smartphones that are using old dated operating systems for some iPhones and Android devices. If reports are to be trusted, WhatsApp will not function on devices that are not running iOS 9 or Android 4.0.3 operating systems.
iPhones that do not run on iOS 9 will not able use to WhatsApp on their phones from 2021. The iPhone models include all iPhone 4 and earlier models. However, if one has iPhone 4S, iPhone 5, iPhone 5S, iPhone 5C, iPhone 6, and iPhone 6S, one will have to update their phone to iOS 9 or later if one wants to continue using WhatsApp.
The iPhone 6S, 6 Plus, and iPhone SE first generation can be updated to iOS 14. Similarly, Android phones that do not have Android 4.0.3 will lose access to WhatsApp. This includes models like HTC Desire, LG Optimus Black, Motorola Droid Razr, Samsung Galaxy S2.