This yearbetking, when Apple announced iOS 18, the latest version of its mobile operating system, most of the attention went to its slate of new artificial intelligence features.

But a lesser-noticed change in iOS 18 — a tweak to an obscure feature that allows users to share their contact lists with various apps — may wind up being more important.

That’s because “contact sync,” as the feature is known by some developers, has played a critical role in the growth of many social and messaging apps for the past two decades. It’s how apps like Instagram, WhatsApp and Snapchat were able to find their footing, by quickly connecting millions of iPhone users to people they already knew, and suggesting other users for them to follow. That early momentum helped kick-start their viral growth, propelling them to the top of the App Store charts.

Now, some developers are worried that they may struggle to get new apps off the ground. Nikita Bier, a start-up founder and adviser who has created and sold several viral apps aimed at young people, has called the iOS 18 changes “the end of the world,” and said they could render new friend-based social apps “dead on arrival.”

That might be a little melodramatic. I recently spent some time talking to Mr. Bier and other app developers and digging into the changes. I also heard from Apple about why it believes the changes are good for users’ privacy, and from some of Apple’s rivals, which see it as an underhanded move intended to hurt competitors. And I came away with mixed feelings.

On one hand, I’m sympathetic to the uphill battle faced by any developer trying to build a new social app today. The contact-sharing changes in iOS 18 will undoubtedly make it harder for some fledgling apps to break through. And in a world where it’s harder for smaller apps to succeed, incumbents like Facebook and Instagram — which already have network effects, and don’t have to ask existing users for permission to keep collecting their contacts — obviously stand to benefit.

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