What does "no visual interruption" for notification importance mean?
No visual interruption means that the notification won't peak at the top of your screen. It will still show up in the notification bar.

https://www.androidcentral.com/notification-channels - This article further confirms that setting notification importance to Low just means you will not be getting any indication that you have received a notification, the notification will still show up in the Notification Bar though. 

The only way a notification would not show up in the Notification Bar would be if you completely disabled notifications for an app.