Is it possible to create a simple application for the latest iOS (18.6) on Xcode 12.4?

No.

The issue here is that last bit, publishing it to the App Store. App Store regularly updates its requirements as to what version of Xcode you can use to build App Store apps. You can view those in Developer > News > Upcoming Requirements. The latest requirements are from Apr 2025, which state that your app must be built with Xcode 16 or later.

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

Thanks!