Can't run PhonePe on my iPhone 6s Plus

You can run PhonePe on your iPhone 6s Plus. That app "requires iOS 14.0 or later", and an iPhone 6 Plus can run the latest release of iOS 15.

You cannot use PhonePe to make or receive UPI payments on your iPhone 6s Plus. The people who set the rules for the UPI payment system do not allow UPI payment apps to make or receive UPI payments on iPhones running anything lower than iOS 17. Your iPhone 6s Plus is incapable of running iOS 17 or higher. In effect, the people in charge of the UPI payment system have abandoned your phone.

I'm guessing that PhonePe has some functions that do not involve UPI payments, are not subject to the UPI rules, and can be carried out on iOS 14, 15, and 16. That's probably the only reason that the vendor has not raised the minimum iOS version to iOS 17.0 – to let you continue to use those other, non-UPI functions.

Because the app developer requires a device running a version of ios that your phone is too old to run.