Is it possible to make a Game Controller vibrate using Xcode?

Apple's response to my feature request via Feedback Assistant:

Thanks for your feedback! For now, this issue behaves as intended.

We love rumble in the Xbox Wireless Controller and Sony DualShock4 controllers, too - we think it would be great in games on iPhones, iPads, tvOS, and macOS, too.

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Looks like this is finally possible! There’ll be a talk about this on Wednesday. I will update this answer as soon as it’s available.

According to Apple's Game Controller Programming Guide, this is not a supported way of interacting with controllers.

Understanding the Controllers Supported by Apple

Apple has created specifications for distinct kinds of MFi game controllers.

Although specific controllers vary, many common characteristics must be implemented strictly according to the specification.

The extended control layout contains the following controls:

• Four analog face buttons arranged in a diamond on the right side of the controller (labeled A, B, X, and Y)
• An analog directional pad on the left side of the controller
• Two analog thumbsticks on the left and right sides of the controller
• Two analog shoulder buttons (labeled L1 and R1)
• Two analog triggers (labeled L2 and R2)
• A button to pause and resume gameplay

The Siri Remote has its own micro control layout.

• An analog directional pad on the top of the remote
• Two digital buttons (A,X)
• One button to pause and resume gameplay

If you directly communicate with a given controller over Bluetooth or similar, you could directly issue controller-specific commands (such as vibrate). Obviously, this would be vastly more complex as you would essentially have to re-implement the GameController framework yourself, listening to commands in an event loop and responding to these in your app. Communication protocols to the controllers likely varies between different makes and models as well, adding even more complexity and cost to the development.

Your best bet is to submit a feature request directly to Apple, via Feedback Assistant.