Thirty years ago, a man stood in front of a column of tanks, halting their hulking passage from Tiananmen Square a day after the bloodshed of June 4. The man has never been identified. But images of his defiance became a symbol of protest against the powerful around the world—except in China, where they have been censored and banished from public memory.

“Tank man” images are ruthlessly excised from Chinese social media, according to monitoring services. When the British journalist Louisa Lim was researching a 2015 book on...