confirm-nonexistent-file-or-buffer is a variable defined in `files.el'.
Its value is

Whether confirmation is requested before visiting a new file or buffer.
If nil, confirmation is not requested.
If the value is `after-completion', confirmation is only
requested if the user called `minibuffer-complete' right before
Any other non-nil value means to request confirmation.

This affects commands like `switch-to-buffer' and `find-file'.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.1 of Emacs.