As far as I understand the release notes, WhatsApp will continue to work on iOS 9.x.
But it can no longer be upgraded on this iOS version.
Also the release notes state that iOS 10 is recommended, NOT required.
The latest version of WhatsApp for iOS 9.x will probably still run for some time.
faq.whatsapp.com
But it can no longer be upgraded on this iOS version.
Also the release notes state that iOS 10 is recommended, NOT required.
The latest version of WhatsApp for iOS 9.x will probably still run for some time.
About supported operating systems | WhatsApp Help Center
faq.whatsapp.com