“THE COMPLETE List of Trump’s Twitter Insults (2015-2021),” reads the headline of a New York Times collection of the now ex-president’s most aggressive missives. The compilation is a disturbing reminder of the vitriol that flowed freely from the @realDonaldTrump handle for years — much of it breaking the rules that platforms decided suddenly in recent weeks to enforce. Which raises twin questions: Why now, and what now?

Before President Donald Trump, social media sites didn’t have exceptions to their rules for world leaders because world leaders didn’t tend to harass private citizens or incite mobs to armed insurrection. Mr. Trump changed all that, and as he did so, the sites changed with him: crafting the so-called public interest exemption to keep newsworthy posts in citizens’ view; then scaling it back to prohibit some offenses even from elected officials; then hiding some posts behind labels; then finally instituting suspensions.

Ungenerously, this could be called flying by the seat of one’s pants. More generously, online platforms were responding to the context of offline life. They saw after the storming of the Capitol on Jan. 6 that harm was being done, so they acted. Yet legitimate as this exercise of power may have been, it was also arbitrary. The two tweets that Twitter cited as sufficiently incendiary to justify Mr. Trump’s permanent ban had hardly more fire and fury to them than so many others he had gotten away with. And it was unclear that he risked being banned forever until, suddenly, he was — just as Facebook’s block, “indefinitely and for at least the next two weeks until the peaceful transition of power is complete,” is unprecedented and ungrounded in policy.

Now activists around the globe are clamoring for similarly stringent remedies to be applied to other leaders who urge the destruction of their enemies or spread disinformation about vaccines, whether it’s Ayatollah Ali Khamenei of Iran or President Jair Bolsonaro of Brazil. Platforms must respond here, there and everywhere by applying the lessons they learned from the debacle of the past four years with explicit terms of service and transparent appeals processes.

To do so, they must answer thorny questions about the role of private companies in policing public figures: Which areas of policy ought to include exceptions and which ought to include no exceptions at all? How do companies factor the context of offline life in to their online decisions? What are the punishments for repeat offenders, progressing from the removal of individual pieces of violating content to account suspension to, eventually, account removal? These all help solve a broader conundrum: When is the good to democracy served by allowing a post or an account to stand outweighed by the harm to democracy, or life, or liberty?

Mr. Trump deserved to be booted from these sites after years of abuse. Someday, when the risk is judged lower, he should be permitted to return to them — at which point he, like the rest of us, will also deserve to know what the rules are and what will happen if he breaks them.

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