F9 keybind isn't in settings, rebindable, and nothing can be unset.

Pressing F9 while not being in the settings just kicks you back to the character select without any confirmation.
This shouldn't be allowed.
It is also not in the options to be rebound...

Also, you cannot Unset any keybinds; you're forced to set them to something out of the way.

This isn't good, but also makes no sense when a bunch of keybinds are defaulted as Unset; meaning it's possible.

Backspace can't be keybound AND doesn't unset the keybind.

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tl;dr: No keybind, let alone a hidden one that can't be changed, should ever kick you back to any main menu, especially without confirmation (Even something like subtle text counting down as you hold it saying you're going back to the main menu).

Especially in a branched game with hold-overs, that evolved into something the first game wasn't; respawning mobs, etc.