What's the minimum SDK version for new iOS apps? Is there a page on the Apple website listing the minimum iOS SDK required for new app submissions?
The Apple Developer news feed is where Apple announces all hard deadlines on SDK requirements that are not enforced when you submit an app. Watching here, you can prepare for future requirements before they are enforced.

https://developer.apple.com/news/

Today (October 2020) it’s the iOS 13 SDK and iOS 14 was announced as the mandatory minimum in April 2021.

Starting April 2021, all iOS and iPadOS apps submitted to the App Store must be built with Xcode 12 and the iOS 14 SDK.