You cannot find this on the official App Store.
Even if you manage to sideload the IPA, the verify your app version. If the version number is below WhatsApp’s server-side cut-off (currently around iOS 10/11 minimum), activation fails.
If you are attempting to revive an old device, these are the common methods used by the legacy community:
Historically, some users bypassed restrictions by and using tools like App Admin to downgrade, but as of 2024–2026, most of these workarounds have failed because WhatsApp has completely cut off those older client versions from their backend servers. WhatsApp On IOS 7.1.2: What You Need To Know - Ftp