When the user kill my app, I need to make an API call to notify my backend.

The solution I found is to blocks the main thread with a semaphore so the HTTP call can be made before app destruction:

func applicationWillTerminate(_ application: UIApplication) {
    let semaphore = DispatchSemaphore(value: 0)

    // Some async API call
    URLSession.shared.dataTask(with: request) { _, _, _ in
        semaphore.signal()
    }.resume()

    _ = semaphore.wait(timeout: .now() + .seconds(2))
}

It looks like a bit "hacky"

Is there a better way to achieve this ?

Could this cause an App Store rejection ?