I want to know if Buddhism was practised in Kerala, India around 500CE. When did it lose popularity and why?
No it was not practiced. In Kerala very minor group of people follow the Buddhism. Buddhism probably flourished for 200 years (650-850) in Kerala. The Paliyam Copper Plate of the Ay King, Varaguna (885-925 AD) shows that the Buddhists benefited from royal patronage in the 10th century. Hope this helps..