Removing Annoying Alerts By Setting Notification Alert Styles
Many apps will give you alerts that appear in the upper right corner of your Mac screen. You can set whether these alerts need you to click on them to dismiss, or they dismiss themselves automatically, or they don't appear at all. The settings for this are in System Preferences, under Notifications.
Alerts appear but cannot find appropriate app to deactivate. ie: install Mojave alert.
EMMIE: That would be a system alert. This tutorial is about how to control alerts from apps.