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Every time you boot a computer capable of upgrading to High Sierra, macOS will pop up at least one notification a day exhorting you to move up. Here's how to turn them off for good, with one Terminal command.
While we at AppleInsider generally recommend upgrading to High Sierra, there are some cases where you might not want to. A home server setup immediately comes to mind, as does a 4,1 Mac Pro with the firmware flashed to 5,1.
Here's how to stop them on El Capitan and Sierra, courtesy OS X Daily.
Open the Terminal in the Utilities folder.
Enter sudo mv /Library/Bundles/OSXNotification.bundle ~/Documents/ and authenticate with your password. Depending on your OS revision, and user permissions, you may or may not get the warning as shown.
The command will execute unceremoniously. Reboot and you won't get update notifications again!
This procedure doesn't preclude installing High Sierra, you can still select it from the App Store. But, it will prevent the notifications from bothering you on a machine that you aren't planning to upgrade — or just can't.