To the best of my knowledge, you cannot use any app to make or receive UPI payments on an iPhone 8 Plus.
My understanding is that NPCI (National Payments Corporation of India) asked for an extra security feature to help combat SMS-related fraud. For iPhones, this feature first appeared in iOS 17. NPCI apparently values it so much that they have made a rule that UPI payments must take place on iOS 17 or higher.
When NPCI and the banks say "Jump!", UPI app developers must say "How high?". Any app developer who doesn't risks losing the ability to participate in the UPI system. Then the app developers say "Jump!" and it is your turn to say "How high?".
There may be some financial apps that do not require iOS 17, that contain a mixture of UPI and non-UPI features, and that will let you use non-UPI features on versions of iOS earlier than 17. But as far as I know, even those apps must refuse to perform UPI transactions on those earlier versions of iOS.
Since the iPhone 8 Plus cannot update past iOS 16, the bottom line is that the people running the UPI system have abandoned your iPhone. Basically, they expect you to buy a new or newer phone if you want to keep making and receiving UPI payments on a phone.
