The notification that appears when a Shortcuts automation runs can be enabled or disabled directly within the Shortcuts app’s Automation settings.
This is not a new feature introduced in a recent version of iOS. The option to control whether an automation displays a notification when it runs has been available ever since personal automations became part of the Shortcuts app on iOS.
To change it, open Shortcuts → Automation, select the relevant automation, and look for the notification-related option. You can turn the notification off if you want the automation to run silently, or leave it enabled if you want to be informed whenever the automation executes.
Therefore, if an automation suddenly starts displaying notifications, the first thing to check is the notification setting for that particular automation rather than assuming that iOS has enabled the notification automatically.