Turning on/off Airplane mode on iPhone X from a desktop computer Is there a tool similar to the Android debug bridge (adb) for iOS? Specifically, if connected to a computer via USB, is it possible to turn on and off Airplane mode with some command, interface or tool?

I have tried libimobiledevice. However, they don't have support for Airplane mode toggling.
libimobiledevice (and technically, usbmuxd as well), like you mentioned, is the functional equivalent of adb in the iOS world, in the sense that it allows you to communicate with your iOS devices over USB.

libimobiledevice doesn't  provide any API that I know of which allows you to toggle Airplane mode.

However, what you can use is UI Automation on an iOS device. You can create a script which would simulate a user opening the Settings app and toggling Airplane Mode.

If your machine is a Mac, you can use Appium (a framework which builds on top of UI Automation). In Appium, you'll first have to create a test automation session, and then toggle Airplane mode. There is sample code on the Internet which explains how you can do this.

If your machine is not a Mac, you can still use Appium but with a twist - you can use commercial tools like Appium Studio, Quamotion WebDriver or xcuitrunner.