dyld: Library not loaded SwiftUI when app runs on iOS 12 using @available(iOS 13.0, *)

Turns out this is a known issue and apple introduced a new build setting flag to handle it

Apps containing SwiftUI inside a Swift package might not run on versions of iOS earlier than iOS 13. (53706729)

Workaround:

When back-deploying to an OS which doesn't contain the SwiftUI framework, add the -weak_framework SwiftUI flag to the Other Linker Flags setting in the Build Settings tab. See Frameworks and Weak Linking for more information on weak linking a framework. This workaround doesn't apply when using dynamically linked Swift packages which import SwiftUI.

Adding -weak_framework SwiftUI to Other Linker Flags fixed my issue

You can also mark SwiftUI.framwerk as optional in Build Phases. Detailed Instruction below.

1. Select your project file in the Xcode navigator.
2. Select your app target (or whichever target you're using SwiftUI from).
3. Select the “Build Phases” tab, then “Link Binary With Libraries” section.
4. Click the + button under the list that appears.
5. Type “SwiftUI” into the search field of the popup sheet, select “SwiftUI.framework,” and click “Add.”
6. On the new row in the table, click the “Required” popup button in the last column. Change the value to “Optional.”