From "Distributing apps using a third-party payment provider in South Korea" posted Thursday on Apple's Developer Support page:
The Telecommunications Business Act in South Korea was recently amended to mandate that apps distributed by app market operators in South Korea be allowed to offer an alternative payment processing option within their apps. To comply with this law, developers can use the StoreKit External Purchase Entitlement. This entitlement allows apps distributed on the App Store solely in South Korea the ability to provide an alternative in-app payment processing option. Developers who want to continue using Apple’s in-app purchase system may do so and no further action is needed.
Those who want to use a different payment system will need to enable the entitlement in Xcode, use required StoreKit APIs, and submit a separate app binary for iOS and/or iPadOS that is distributed solely on the App Store in South Korea. Apple will review this South Korea-specific app to ensure it complies with the terms and conditions of the entitlement, as well as the App Store Review Guidelines and the Apple Developer Program License Agreement...
Configuring and enabling the entitlement in Xcode
After you’ve received an email confirmation that the entitlement has been assigned to your account and you’ve configured your app’s App ID in Certificates, Identifiers, and Profiles to support this entitlement, you’ll need to update your Xcode project, entitlements plist file, and info.plist file to list the entitlement and metadata. The entitlement is compatible with devices running iOS 15 and iPadOS 15 or later.
- In the Project navigator, select the .entitlements file. In Xcode 13, the filename is prefixed with an icon.
- In the entitlements plist file, add a new entitlement key pair by holding the pointer over the Entitlements File row and clicking the Add button (+).
- Provide the following values for the entitlement:
- Provide the required metadata in your Info.plist file as described in Updating your Info.plist file.
On the next build to your device or distribution request in Xcode Organizer, Xcode will detect that the .entitlements file and cached provisioning profile don’t match, and will request a new provisioning profile based on the latest app ID configuration to complete the code signing process.
Updating your Info.plist file
Each entitlement has unique requirements for the data that must be entered into your app’s info.plist file. For details on managing your app’s info.plist file, view documentation.
StoreKit External Purchase
Select the Info.plist file from the Project Navigator in your iOS target.
Provide the following values for this entitlement:Key: SKExternalPurchase
Type: Array of String
Value: KR (The single ISO 31566-1 alpha-2 country code value for South Korea.)
My take: Apple's back-end -- the view developers get-- is rarely pretty. This one is downright ugly.