When a shopper in New York, say, plucks a Milky Way bar from a grocery store shelf, that shopper becomes the final link in a long chain that might have started on a patch of land in Ghana, where a tropical forest recently stood. About 80 percent of the trees razed each year in the tropics are cleared to make space for growing cocoa, soybeans, palm oil and cattle that are the raw materials for chocolate, cereal, leather seats and thousands of other products. Ten years ago, some of the world’s largest companies, including Coca-Cola, Kellogg’s, Walmart and Mars, pledged to change their practices to help end deforestation by 2020. Some, like Nestle and Carrefour, went even further, saying they would eliminate deforestation from their supply chains altogether. The 2020 deadline arrived, and some companies reported advances toward their goal. No company, however, could say it had eliminated forest destruction from its supply chain.