While official support has ceased, the Android ecosystem allows for "Sideloading" (installing applications manually without a store). To install WhatsApp on Android 4.4.4, the user must locate a legacy version of the application that predates the API 21 requirement.
Cause: WhatsApp has deprecated the backend server for your app version. This usually happens 6-12 months after the last update. Download Whatsapp For Android Version 4.4.4
Fix: Unfortunately, there is no fix. You will be forced to upgrade to an incompatible version. At that point, your device has truly reached end-of-life for WhatsApp. While official support has ceased, the Android ecosystem
Users currently on Android 4.4.4 have limited options if they wish to use WhatsApp: While official support has ceased