WhatsApp utilizes a Client-Server architecture. Even if you have a client (app) version from 2018 that supported iOS 9, the WhatsApp servers have been updated.

If your iOS device is running iOS 9.3.5, you may not be able to download WhatsApp from the App Store, as the app may not be compatible with your device's operating system. In this case, downloading WhatsApp IPA for iOS 9.3.5 is a viable alternative.

However, this approach hits three major walls:

that still support iOS 9: