SHANGHAI—Alibaba Group founder Jack Ma made his first public appearance since October 2020 on Jan. 20, speaking to a group of teachers by video, and easing concern about his unusual absence from the limelight and sending shares in the e-commerce giant surging. Speculation over Ma’s whereabouts has swirled in the wake of news this month that he was replaced as a judge in the final episode of a reality TV show, and amid a regulatory clampdown by Beijing on his sprawling business empire. The billionaire, who commands a cult-like reverence in China, hadn’t appeared in public since Oct. 24, 2020, when he blasted China’s regulatory system in a speech at a Shanghai forum. That set him on a collision course with officials and led to the suspension of a blockbuster $37 billion IPO for Alibaba’s financial affiliate Ant Group. Until then, Ma had often appeared in public, speaking at conferences …