Some developers have created “Web wrappers” for old iOS versions. For example, a modified app that loads web.whatsapp.com inside a UIWebView.
Here is the current reality of using WhatsApp on such an old firmware and your best options for moving forward. The Reality Check: Does WhatsApp Still Work? Ios 4.2.1 Whatsapp Ipa Download
— support for iOS versions below 10 (and later below 12) ended several years ago. Even if you find an old IPA file (e.g., WhatsApp version 2.12.x or earlier), you will not be able to activate or use it because WhatsApp’s servers now require newer API versions and security protocols. Some developers have created “Web wrappers” for old
Technical Analysis & Retrospective