I'm using Kubuntu 18.04.1 on a System76 laptop using Xorg and there is a keyboard shortcut that I can't get rid of. ALT and any of the function keys switches the virtual tty (instead of ctrl-alt-F# like I'm used to). This conflicts with common shortcuts I use such as Alt-F4 to close a window. Furthermore, ALT- will switch to a different tty (which conflicts with the BACK shortcut I use when browsing the web).
I've checked the (many) plasma keyboard shortcut settings and couldn't find anything (in fact, Alt-F4 is bound to close windows in kwin). I found this question: How to config hotkeys for switching between consoles in ubuntu and that let me enable using Ctrl-Alt-F# to switch TTYs, but the ALT-F# keybinding persists. The same keystrokes work in the other virtual consoles where X isn't running, so I think this isn't a KDE/Plasma issue, but I don't know.