Have you had practical experience using subscription systems or mechanisms?

Yes, practical experience with subscription systems or mechanisms is usually associated with the "Observer" pattern, where one object subscribes to events of another.

For example, in iOS, NotificationCenter is often used to subscribe to system or user notifications:

NotificationCenter.default.addObserver(self, selector: #selector(handleNotification), name: .myNotification, object: nil)

@objc func handleNotification(_ notification: Notification) {
    print("Notification received")
}

You can also use Combine — Apple's framework for reactive programming, where subscription occurs to a Publisher:

let publisher = Just("Hello")
let subscription = publisher.sink { value in
    print(value)
}

Thus, subscription mechanisms allow reacting to events or data changes asynchronously and conveniently managing data streams.