What does Apple's new App Store requirement mean
Minimum SDK ≠ minimum OS version.
You will have to compile your app with the SDK ≥ 12.1, but your Deployment Target can be lower than that. Apple wants to &quot;force&quot; you to prepare the UI for iPhone XS Max.
Here's more info about the SDK and Deployment Target on StackOverflow.
Simple explanation from Big Nerd Ranch:

SDK stands for “Software Development Kit,” which is the set of libraries and headers for a particular OS version. This is the version of Apple’s APIs that you compile and link against. The SDK describes the set of API available to you. Linking against the iOS 9 SDK means you can use any API that comes with iOS 9. You won’t be able to directly use stuff introduced in iOS 10. Modern Xcodes are tightly coupled to the SDKs for the latest OS versions, so if you upgrade your Xcode, you will be linking against a newer version of the SDK.
The other version number is the Deployment Target. This declares the oldest OS version your app will support. How far back you decide to support is a business decision based on how much work you are willing to do for customers on older versions of the OS.