Welcome to the forums @ralph
So, Apple’s iOS is a closed source operating system and gets frequently updated as critical bugs are fixed. Nighthawk relies on certain iOS frameworks which compiles with specific versioned releases of iOS. We expect iOS users to be running the latest available version of iOS for all the security benefits, especially when using a mobile wallet app.
iOS fixes range all the way from the kernel to UI level. Apple has done a good job to maintain backwards compatibility of iOS releases for the last 4-5 generations of hardware. (Apple iOS 14.6 Release: Should You Upgrade?)
Here is a list of security fixes in iOS 14.5: About the security content of iOS 14.5 and iPadOS 14.5 - Apple Support
Android does have major security holes in the older versions too, but Android is open source, and it allows OSS projects to release patches in dependencies which enables apps to be backwards compatible. Still, it is recommended to use the latest available version of Android as well.
Thanks for your feedback Ralph, this prompts me to make a supported devices/OS versions page for Nighthawk.
Update: Here is the supported devices page https://nighthawkwallet.com/supporteddevices/