Call 2018 the “Year of Deplatforming.” The internet was once celebrated for allowing fresh new voices to escape the control of gatekeepers. But this year, the internet giants decided to slam the gates on a number of people and ideas they don’t like. If you rely on someone else’s platform to express unpopular ideas, especially ideas on the right, you’re now at risk. This raises troubling questions, not only for free speech but for the future of American politics and media.

The most famous victim of deplatforming is, not coincidentally,...