Parler can return to the App Store if it does this one thing
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UPDATE:The CEO of Parler, the right-wing social media app suspended from the Google Play Store and the Apple App Store, told Reuters this evening that the app may never return. CEO John Matze says that he doesn't know when or if Parler will give in to Apple's demands. The site has over 12 million users.
According to AppleInsider, Apple CEO Tim Cook explained today on a taped interview on the CBS This Morning show what Parler needs to do to be re-listed in the App Store. Answering a question about Parler's CEO, Cook pointed out that "We suspended them, we did not ban them. We have Terms of Service for our App Store, and some of those Terms of Service he's in violation of. All we're asking is he meet the Terms of Service."

Tim Cook says that by following Apple's ToS, Parler can return to the App Store
So on the face of it, a reinstatement could be as simple as Parler agreeing to follow Apple's Terms of Service to the letter. But while this might sound simple, it might not be for the social media app. "Parler has some issues with moderation," Apple's CEO said. "Our hope is that they do that and get back on the store." For example, the day that Google removed the app from the Play Store, Apple gave Parler 24 hours to remove messages promoting hate and violence and asked it to come up with better ways to moderate the app. Parler refused to make these changes leading Apple to remove the app from the App Store. There is no indication that Parler would be more likely to follow Apple's ToS with this second chance.
Things that are NOT allowed: