Communities like r/LegacyJailbreak often host verified links to older app versions.
Enter your and password (this is required to sign the app certificate with Apple). Click Start and wait for the process to complete.
An is an iOS Application Package file. By obtaining an older version of the Messenger IPA that was compatible with iOS 9, you can "sideload" the app onto your device, bypassing the App Store restrictions. Step-by-Step Guide: Installing Messenger on iOS 9.3.5
Apple transitioned from a 32-bit architecture to a 64-bit architecture with newer iOS releases. iOS 9.3.5 represents the final operating system for 32-bit Apple devices.