When is iOS 17 the only option to update?

Currently, users can update iOS 16.7 and optionally install iOS 17. At what point is iOS 17 the only version to update? I don't want to have over half the app users on the iOS 16.7 version and not able to update my app. Would they still be able to install the "outdated" iOS 16.7 version or does the app store just not let them find the app if I have updated to iOS 17?

When is iOS 17 the only option to update?
 
 
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