Our site is at our 3/3 custom close reasons, so one of the top three in that list would need to be first deactivated to add a new reason. Feel free to propose replacing on of our reasons here in meta. Personally, I think we want to give visitors more direction when possible, rather than an overly generic close reason, so that it doesn't come across as a "go away" (even if that's what many of us mean).
The generic text in the top banner is used in the following scenario:
This is the top-level reason that appears when there are no custom close reasons. When a question is closed with multiple community-specific reasons but no reason is visible (in the case of migrations and "other"), this post notice will display on the question.
Our banner is currently set to the site default:
This question does not appear to be about $Topic within the scope defined in the [help/on-topic].
We also have the flexibility to customize that message for the post owner and for privileged users if needed.
To see the details behind what happened on any question, click the clock icon under the question vote buttons to see the activity on the question. That will show individual close votes, along with reviews and other related actions.
(the clock icon is the bottom one in the screenshot above)