The scaling back of Twitter’s efforts to define dehumanizing speech illustrates the company’s challenges as it sorts through what to allow on its platform. While the new guidelines help it draw starker lines around what it will and will not tolerate, it took Twitter nearly a year to put together the rules — and even then they are just a fraction of the policy that it originally said it intended to create.

Twitter said it had ratcheted down the policy’s scope partly because it kept running into obstacles. When the company sought users’ feedback last year on what it thought such speech might include, people pushed back on the proposed definitions. Over months of discussions late last year and early this year, Twitter employees also worried that such a policy might be too sweeping, potentially resulting in the removal of benign messages and in haphazard enforcement.

“We get one shot to write a policy that has to work for 350 million people who speak 43-plus languages while respecting cultural norms and local laws,” Mr. Peterson said. “It’s incredibly difficult, and we can’t do it by ourselves. We realized we need to be really small and specific.”

Twitter unveiled its new policy ahead of a social media summit at the White House on Thursday that is likely to thrust it and other Silicon Valley companies under the spotlight for what they will and won’t allow. For the event, Mr. Trump has invited conservative activists who have thrived on social media, such as Charlie Kirk, president of Turning Point USA, which advocates limited government and other issues. Many of the attendees have accused social media companies of anti-conservative bias.

Twitter declined to comment on the meeting.

In the past, Twitter has focused its removal policies on posts that may directly harm an individual, such as threats of violence or messages that contain personal information or nonconsensual nudity. Under the new rules, the company is adding a sentence that says users “may not dehumanize groups based on their religion, as these remarks can lead to offline harm.” Twitter said that included any tweets that might compare people in religious groups to animals, insects, bacteria and other categories.