Diclofenac has been banned in India for veterinary use since 2006. Another NSAID that is not deadly to vultures is recommended for such use. Not sure why Guardian ran this story now as this is old news. The real crisis with plunging vulture population goes far beyond Zoroastrian burial rites. See https://en.wikipedia.org/wiki/Indian_vulture_crisis
That Wikipedia article is what made made curious, as it says "drugs like diclofenac..." From that small bit, it seems like the many NSAID's given to humans such as ibuprofen or asprin could be issues. Even diclofenac seems to have some human use.
I suspect stronger drugs such as codeine etc are used for palliative care of humans in their last stages. Not sure of their effect on vultures. Parsi numbers have never been high and gradually decreasing (now about 70,000 in India), also they mainly live in relatively few places, most in Mumbai and nearby. The vulture population dropped from 40 million to a few hundred so the primary cause must be from their non-human diet.
Low fertility rates (like in most well to do communities), interfaith marriages (if you marry a non-parsi and your kids are not considered parsi), aging population, migration out of India.