If you still want to try for archival or experimental purposes, there is one "classic" workaround that sometimes works for older apps, provided you have used WhatsApp with your Apple ID before: The "Purchased" Method: on your old device, go to
Q: Can I download WhatsApp old version for iOS 9.3.6 directly from WhatsApp? A: No, WhatsApp doesn't provide direct downloads for older app versions. whatsapp old version for ios 9.3.6 download
Apple allows users to download the of an app if they previously downloaded it. This is called the “purchased” method. If you still want to try for archival
If you must keep using this device: