India is really frustrating for tourists right now... I was there earlier this year, and there were several things I just couldn't do without UPI... Some restaurants just didn't accept cash so I couldn't eat there... Bus tickets had to be booked online with UPI... I think they're moving towards opening it up, but when I was there it was completely impossible for a foreigner to use UPI.
Caveat - only available to tourists from a G20 country, and they need to register while exiting at an airport. This doesn't work for our neighbouring countries - Bangladesh/Sri-Lanka/Nepal which have the highest number of visitors to India.