There are several Android and iOS phones that may not be able to run the app as the company will withdraw support for older versions of OS from next year. According to the information on the WhatsApp FAQ section, WhatsApp will only be compatible with phones running the Android 4.0.3 operating system or newer as well as iPhones running on iOS 9 and newer.
All iPhone models up to the iPhone 4 will lose support for the messaging platform in the next few days. The iPhone models include the iPhone 4S, the iPhone 5, the iPhone 5S, the iPhone 6, and the iPhone 6S.
For Android, devices including HTC Desire, Motorola Droid Razr, LG Optimus Black, and the Samsung Galaxy S2 will lose WhatsApp support as 2020 ends. The instant messaging platform will keep the app running for select phones with KaiOS 2.5.1 OS or newer, including JioPhone and JioPhone 2.
Ref: Business Insider.