iOS 15 and PadOS 15 are both confirmed compatible with ChartBuilder.
The latest version of Playback can be installed on devices that support iOS 9 and above, which include:
iPhone 4S and above
iPad Pro
iPad Air 1 and above
iPad with Retina display (3rd and 4th generation)
iPad Mini (all generations)
iPod Touch (5th and 6th generation)