And if not, could this be added in the future? Seems like it is from aosp and not the isp, but I might be wrong.

if you mean if it's possible to disable all possible device notification, then mostly likely not. But for all user installed apps, you can disable the notification. If i remember correctly, phone app need notification and so does the settings app. OR if u need to temperately disable all notifications, you can just turn on Do Not Disturb mode

just disable notifications for all the apps you install, or use do not disturb.