As operating system versions advance, WhatsApp is adjusting the minimum version required to avoid the extra work of weighing down backward compatibility. The latest version of WhatsApp available on the App Store removes support for iOS 9: It requires iOS 10 or higher.
Whatsapp now requires iOS 10 or higher
WhatsApp periodically updates the minimum requirements of its application in terms of the operating system version. In the Apple operating system, the minimum version required was so far iOS 9, but the latest version published in the App Store leaps iOS 10, i.e., you can not install it if you have iOS 9.
This does not mean that WhatsApp will stop working immediately on mobiles with iOS 9, but that these devices will be stuck on the previous version and will not receive any more support, updates, or new features. In iOS 9, WhatsApp is left to its own devices, so it will most likely stop working sooner or later. WhatsApp usually displays a warning message when you are using a very old version.
In practice, you will need to have a fairly old iPhone to not have a higher version of iOS available. Specifically, iPhone 4s or earlier are those whose latest version of the operating system is iOS 9 or earlier. The iPhone 4s was released in 2011, ten years ago. On Android, the minimum version is still 4.0.3, also from 2011. According to Apple, less than 2% of active devices use a version before iOS 13.