[abtom]

From what I understand it wasn't just this. WhatsApp's initial implementation of UPI was that of a walled garden. Payments could be sent/received between WhatsApp users but you couldn't send/receive payments to/from another UPI app user. They did this by hiding the UPI ID from the user. Along with WhatsApp's mass user base, this would have resulted in fragmenting the ecosystem.

I have enrolled in the beta program for WhatsApp via Google Play so I had the payment option In the new implementation since a very long time (can see a payment from July 2018). In their new implementation they have exposed the UPI ID so compatibility with other UPI apps is ensured. For transactions with other WhatsApp users you need not bother with UPI ID. This dual approach is the same as that of Google Pay.