Comment 9 for bug 552425

You might be right: if I look at the United State keyboard on Wikipedia, there is no key between the left shift and the "z" key:
http://en.wikipedia.org/wiki/Keyboard_layout#QWERTY-based_layouts_for_Latin_script

Other layouts, like the United Kingdom keyboard, have a key between the left shift and the "z", but on the other hand, they lack a key above the return key compared to the US layout. As the compact layout is intended to be used with various languages, the best probably is to keep both keys.

Onboard supports the creation of custom layouts through xml and svg files; people, that need particular layouts might use this feature and create a custom layout.
(The coming Onboard version 0.99.0 of Onboard is planned to ship two additional layouts geared for mobile devices.)