In 2006, I agreed to release the source code of the x-unikey package (originally developed for Linux) under the MIT license to Apple, so that they could use the code to build the Vietnamese keyboard input in Mac OS X. From Mac OS X Leopard, released in 2007, Apple has integrated UniKey Vietnamese Input Method in the the built-in Vietnamese input of Mac OS. From 2010, the engine has also been integrated to the built-in Vietnamese keyboard in iOS (starting from iOS 4.0). So the UniKey engine is now running in all macOS computers, iPhones, iPads that have the Vietnamese keyboard input.
Picture below shows the Vietnamese keyboard called Simple Telex in macOS, which uses the Unikey Vietnamese Input Method engine.
In iOS, you can find the Apple’s acknowlegdment of the use of UniKey source code in Settings - General - About - Legal - Legal Notices, as shown below.