Given that he stepped away from a day to day role in 2008 and then stepped down as the chairman in 2014, I'm guessing he's focusing on his foundation.
He's still the 8th largest shareholder according to Bloomberg's records(103,000,000 share), which is pretty impressive given that the 7 entities above him are all funds.
Incase you wanted to know who the remaining 12 board members are:
With Gates’ departure, the Board will consist of 12 members, including John W. Thompson, Microsoft independent chair; Reid Hoffman, partner at Greylock Partners; Hugh Johnston, vice chairman and chief financial officer of PepsiCo; Teri L. List-Stoll, executive vice president and chief financial officer of Gap, Inc.; Satya Nadella, chief executive officer of Microsoft; Sandra E. Peterson, operating partner, Clayton, Dubilier & Rice; Penny Pritzker, founder and chairman, PSP Partners; Charles W. Scharf, chief executive officer and president of Wells Fargo & Co.; Arne Sorenson, president and CEO, Marriott International Inc.; John W. Stanton, chairman of Trilogy Equity Partners; Emma Walmsley, CEO of GlaxoSmithKline plc (GSK); and Padmasree Warrior, founder, CEO and president, Fable Group Inc.