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.
Best Answer
Credit for the answer goes to DK Bose who linked the answer by none other than Alan Pope here: https://askubuntu.com/a/929115/248158. But for completeness, I'm putting the answer here as well. The keyboard was set to unicode mode, and by setting it to raw mode, these key sequences were then interpreted as expected.
This is persistent across reboots. And to restore it to how it was before: