https://incometaxindia.gov.in/communications/circular/circul...
Besides this, stakeholders/partners of the NPCI are mandated to offer UPI services. I'm not sure if signing up was optional, but it would honestly be suicidal for a bank not to, given that there are so many competitors offering the same.
I keep appreciating the regulations in India, it is overall supportive of the common man. From say SEBI regulations on derivative markets to digital payments. For eg. credit cards are becoming less frequent, people here won't take out their cards for every other expense. People have become aware of the fact that money at the end is in bank and spending from the bank account hits much more real than a 30 day stream of cash 3-4 times your salary handed to you whenever you need. And so, low income and middle income groups spend wisely and don't get caught in spend cycles above their capacity.
Such an angle goes unappreciated, but hopefully this continues and money still be real and in our pocket.