When configuring notification categories, you can change the Importance to Urgent, High, Medium, and Low. The description for Low says "No sound or visual interruption." What does no visual interruption mean? I want to set certain notifications to the lowest possible, but I still want them showing in the notification bar, so if no visual interruption means that it will disappear from my notification bar, then I'll leave it at Medium.
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.