The TrueDepth camera [that supports head tracking] is supported on iPhone X and later, and iPad Pro 12-inch (3rd generation) and 11" iPad Pro. So you would need one of those for head tracking.
So you're right, I should have specified that older iOS devices are going to be limited natively to use of the head as a switch.
But, also, there are ways to use a traditional or alternative [head, tongue, chin] mouse with those, e.g. the Lightning to USB Camera Adapter (essentially a USB port, that works with any device running iOS9 or later), or the Dock Connector for earlier models, as well as ability switches with interfaces like Tecla. Newer models, of course, have a USB-C port built in.
For those who can use a chin or tongue mouse, I recommend a Tetramouse, which also operates under Bluetooth, which is supported in iOS 13+. Of course, the laptop experience is much better for all alternative mice, and you can make calls there as well via Google Voice, Skype, Facetime, etc. But when on the go, chin controls and ability switches are certainly sufficient to operate iOS devices, depending on mobility.