According to reports, WhatsApp would stop working on older iPhones and Older Android devices from 1st January, 2021.
iOS version upto 8 and Android version upto 4.0.3 would not be able to get further update of WhatsApp, as like every other developers, WhatsApp will be withdrawing support for older hardware & Software.
WhatsApp now will only be compatible with Android KitKat or newer and iPhones running iOS 9 or newer, According to Application FAQ. If you have older phones like this, it’s may be the time for upgrade.
All older models will lose the ability to use WhatsApp in next few days, as 2020 ends. To check your Android or iOS version, go to your settings app and then about phone section.
However, WhatsApp will keep working on devices running KAI OS 2.5.1 or newer, including JioPhone & JioPhone 2.