There used to be an App of Stack Overflow on iPhone. But I don't find it in the App Store anymore.
Is it removed from the App Store?
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The mobile apps are unfortunately discontinued.
You can read the formal post here, quoting the most relevant part:
We are not providing any further support for the mobile apps. There will not be any more builds made for either the Android or the iOS app (even for breaking bugs). We encourage all mobile users to use the responsive website. We hope to continue to incrementally improve the user experience for mobile users.