Enabling the Shield icon on the main screen will prevent your device from receiving operating system updates (i.e. Android 10 to Android 11). When the lock icon in the center of the shield is green that means the Update Shield is Enabled (shown below in the screenshot). The reason for this feature is because oftentimes not everybody wants the next update to their OS. Updating to a new operating system can sometimes break existing applications or configurations you have in place until the developer of those apps updates their compatibility to the new OS update (if needed). This way, disabling updates gives you time to read over changes and see if it will cause any issues to your current device setup. You can also keep an eye on reviews that other people have written about any bad experience they’ve had after updating that could relate to you.
If you’re using Device Owner or ADB method then the Update Shield button will fully disable the update packages within the application manager.
Example of Update Shield ENABLED