Because of this, standard IPA files for WhatsApp will not work on iOS 6. However, some legacy enthusiasts use unofficial community workarounds to keep older hardware alive. 1. Legacy WhatsApp IPAs
Modern WhatsApp requires encryption protocols and server handshakes that iOS 6 simply cannot handle. How it was traditionally done (Archive)
If you absolutely must run WhatsApp on a classic iOS device, your only real option is to or tethered downgrade that device to iOS 8 or 9, where some older but still functional WhatsApp builds may last a little longer. But for iOS 6? The party ended years ago.