WhatsApp doesn't open on Android 2.3.6 - says "switch to a supported phone" Whatsapp say app supported to 2020 but won't open from 2018-02-06
According to whatsapp firm : 

Supported devices
We support all Android phones that meet the following requirements:

Your Android phone is running Android 2.3.3 or later.

Your Android phone is able to receive SMS or calls during the verification process.

You will also need a data plan in order to receive messages when outside the range of a Wi-Fi network.

We only provide limited support to tablet users and we don't support Wi-Fi only devices at this time.