The only legitimate method is the "App Store Purchase History" trick. If you had previously installed WhatsApp on that Apple ID in the past, you can go to the "Purchased" tab in the App Store, find WhatsApp, and hit download. The system will prompt you with: "Download an older version of this app? The current version requires iOS 10 or later." Once you accept, you get the last compatible build (usually version 2.19 or similar).
: Some developers have created specialized clients like "WhatsApp for legacy iOS" that act as a wrapper for the web version on older devices (e.g., iPhone 5 or 4s). These require an active WhatsApp account on a newer, supported "primary" phone to link to. whatsapp old version for ios 9.3.6 download
Here’s why: