Ubuntu – Why does ^C, ^V etc. appear in the terminal when I use the Ctrl+character keyboard shortcut

you can, But you have to go into the settings.

Edit -> Keyboard shortcuts -> find copy and past commands -> Click to alter and enter new combo "ctrl+x, v, c, etc..."

One possibility is that the backslash key has become bound to some readline command (readline is the library that allows in-situ editing of shell commands, for example binding Ctrl+h to the backward-delete-char action).

To test if that might be the case, you can try starting a bash subshell with readline editing disabled

bash --noediting

If the backslash key works in that subshell, then the places to look for the rogue keybinding definition would be /etc/inputrc for the global configuration and ~/.inputrc for per-user configuration. You could also try printing out and searching the currently defined keybindings from the command line using

bind -P | grep '\\\\'

The extra backslashes are required because a literal backslash will appear in the readline configuration as \\, each of which needs a further escape in the shell. Obviously you can only do that in the subshell, if the backslash is being bound in the regular shell.

FYI there is a bind command option to remove a binding, but it is not clear how to input the keyseq argument when it is already bound

-r  keyseq         Remove the binding for KEYSEQ.

For further information see help bind in the bash command line