The People’s Republic of China turns 70 in October and will celebrate with flag-waving and fireworks. But 2019 marks several other major Chinese anniversaries whose public remembrance the Communist Party will suppress—and another milestone whose observance has been surprisingly muted.

Twenty years ago, it was Falun Gong adherents being arrested. In July 1999 the Communist Party branded the spiritual meditation group an “evil cult.” On April 25, 1999, 10,000 practitioners, many of them elderly, had held a silent demonstration...