In the Keyboard settings menu, the "Overlay Menu" action is grayed out, which makes sense as a measure to stop the player removing the bind and being locked out of fixing their mistake. However, it is still possible to unbind this action by binding a different action to [Esc]. 

Once the menu has been exited and gameplay resumed after this change, this results in the player being stuck without any option to reopen to the settings menu.  As a workaround, the player can force quit, reopen the game and then access the keyboard settings from the main menu, but since the Overlay Menu action is still locked, the only recourse is to restore all keyboard settings to default in order to restore their ability to use the menu in-game.