I will be visiting at the end of this month and have been trying to get good information about the current state of picking up a PrePaid SIM. I am intending to get an airtel from Delhi Airport T3 upon arrival. Having read about issues with this here and on Reddit; I've asked our accommodation (in Aerocity) to tell us about it. They say
Pleased to inform you that you can buy new sim card from Terminal 3 only near gate no 3 and 4, you will be needing only Passport and valid Visa to get a new sim. You will face no consequences. .... One more thing, once your sim will get activated you can buy a plan accordingly.
Does this seem accurate?
Of course, I will update with actual experiences next month!