TikTok kept a safety fix from 15 million US users “by design,” a sealed document shows. One was a teenager who died.

In 2021, TikTok changed its algorithm to stop users being flooded with harmful content. But it did not give the safer version to everyone. It held 10% of US users on the old version, about 15 million people at the time. It wanted to test whether the safer feed was less addictive. That is according…