I'd like to know if Apple states the lowest permissible base SDK for submission to the iOS App Store. There was a statement somewhere around 2010 that said you need to use at least SDK 4, but i cannot find this any longer.
Now, before you jump me, I always build against the latest base SDK. The reason i ask this question is because i want facts to counter some customer requests.
Apple seems to have never explicitly required any Base SDK above iOS 3.0 as far, as far I can find, today, either in their app submission guidelines or any official developer email. Thus, there likely is no such link as you require. However there are many indirect hints.
As of 2013-May-01, Apple indirectly requires a Base SDK of 6.0 for submitting iPhone apps by requiring 4" display support and no longer approving any apps where 4" support was added using any Base SDK lower than 6.0 (that latter stated in their enrolled developer forums, login required). This requirement may or may not apply to iPad-only apps, where Apple might still allow submission using a Base SDK of 5.1.
There is a statement (in their enrolled developer forums, login required) from an Apple employee that currently a Base SDK of 7.0 or higher is not required for app submission, but with no date given as to when this may or may not change.
Note that if a developer uses an SDK lower than 7.0 to build an app, they still need to test the app on a iOS 7.x device or risk rejection by Apple if the app fails to run properly on such a device.
The Deployment Target of an app can get set to a much lower OS version. But an iOS Base SDK of 6.0 lacks armv6 support, and thus doesn't allow XCode to build an app with a Deployment Target lower than 4.3 where armv6 support is required.