American companies began to rethink their requirements for face masks after federal health regulators relaxed their guidelines this week, and on Friday Walmart Inc. made the first big move to bend to the new view. The U.S.’s largest private employer said it would no longer require vaccinated workers and shoppers to wear masks in stores and warehouses outside of municipalities that require it. Walmart’s WMT, +0.93% new no-mask policy for its 1.6 million U.S. workers goes into effect May 18, the company said, while vaccinated customers could shop maskless immediately.