How to decline upgrade and get rid of notification badge.

If you want to continue running macOS 26 Tahoe and keep that up to date, but skip the upgrade to Golden Gate for now, then to update to the latest version of Tahoe do the following:

• Open System Settings > General > Software Update.
• Click the (i) next to Other Updates under Also Available.
• Select the update(s) you want, which may include both macOS Tahoe and Safari updates.
• Click Update Now.

Note that you cannot get rid of the badge on System Settings. The only way to do that is to keep your Mac up to date. However, if you're on Tahoe and that is up to date, you should not see the badge. For example, my work Mac (16" M1 Pro MBP) is capable of running Golden Gate (the above screenshots are from that Mac) but still running Tahoe, and has no badge on the System Settings icon.

Regarding the Apple Intelligence components of macOS 27, Apple Intelligence launched with macOS 15 Sequoia (and iOS 18), and since then it has become progressively more integrated with macOS (and iOS). On macOS 26 Tahoe, there was option to turn off Apple Intelligence (though doing so did not remove the 12-14 GB of local storage taken up by Apple Intelligence features). 

On macOS Golden Gate, the toggle above has been removed, and Apple Intelligence is even more tightly integrated with macOS. Consequently, it is very difficult to turn off. Long-time forum member etresoft has written a helpful tip on how to Turn off Apple Intelligence in macOS 27. The title is somewhat misleading, Apple phrases it more accurately in their support article:

Turn off and restrict access to Apple Intelligence features on Mac

As Apple states, you can turn off and/or restrict access to some features of Apple Intelligence, but you cannot turn off Apple Intelligence altogether.

Apple Intelligence features can be restricted in a more comprehensive way by using a declarative configuration, but that requires device enrollment (i.e., management of the Mac with MDM) and that is beyond the typical processes with which most users are familiar. However, there is a script called Remove Mac AI that is available to execute those functions as a local Profile via a terminal command, if you are comfortable using it. At least one long-time member here has done so without issue. Use it at your own risk.

Unfortunately, Apple does not provide a straightforward, supported way to permanently dismiss major macOS upgrade notifications while continuing to receive normal security updates.

You can disable automatic installation of macOS upgrades under System Settings >> General >> Software Update >> Automatic Updates 

This prevents automatic installation, but it does not necessarily remove the red notification badge in System Settings.

Apple's Mobile Device Management (MDM) framework allows administrators to defer certain macOS upgrades for a limited period, commonly up to 90 days, depending on the update and configuration. 

MDM is not restricted to businesses; home users can enrol a personal Mac with a compatible management provider. 

However, this requires a management service and appropriate configuration, making it rather excessive for someone who simply wants to dismiss an upgrade reminder. Furthermore, the deferral is temporary, and MDM does not guarantee that the System Settings badge will disappear permanently. Once the deferral expires, the upgrade may become available again.

For more information, see the HCS Technology Group macOS Software Update and Jamf guide.

There are older workarounds involving Terminal commands, software-update preferences, configuration profiles and third-party utilities, but their effectiveness varies by macOS version. Apple can change or override these mechanisms, and modifying protected system files or disabling update services can interfere with security updates. There is no universally reliable, Apple-supported configuration setting that permanently suppresses major-upgrade notifications while preserving normal security updates.

Disabling Apple Intelligence also does not necessarily remove associated applications such as Image Playground. Some system applications remain installed even when their features are disabled, and attempting to remove protected system components is generally not advisable.

Ultimately, we own the Mac hardware and its storage, but macOS is software licensed to us by Apple rather than owned outright. Apple retains considerable control over the operating system's functionality, update mechanisms and notification behaviour.

The frustrating part is that, even when we deliberately decide not to install a major upgrade, Apple still controls how macOS presents the upgrade notification. We can decline the upgrade, but we have no straightforward, supported way to tell the operating system to stop reminding us.

This leaves users with limited control over an aspect of a computer they have purchased and own. If we have  consciously decided to remain on our current macOS release, why should we have to keep dismissing reminders for an upgrade we do not want?

A simple “No thanks — don't remind me again” option would respect the user's decision without preventing security updates from being installed. That seems like a reasonable feature for Apple to provide.

In short, we can disable automatic upgrades and, in some circumstances, defer them temporarily through MDM. However, there is currently no universally reliable, supported way to permanently remove the major-upgrade notification while retaining normal security-update functionality. 

For most home users, MDM is an impractical workaround rather than a permanent solution.

If you do like the red badge reminder then make an alias of the system settings icon and put that in the dock. It will not include the red marker. Remove the default system settings icon from the dock.

You can't get rid of the red badge. The notifications cannot be stopped but after a while they become much less frequent - until the next update comes out, of course.

Thanks for the thoughtful and thorough reply, I have already done the security update and shut off all AI (everywhere I can find it) in Tahoe, unfortunately there are some apps still cluttering my system that cannot be deleted right now - e.g. Image Playground.app and one other.

The System Setting.app icon eventually doesn't have the notification badge - so long as I don't go into System Settings, but going into the app makes it reappear for some time and I want to get rid of it entirely as I do not plan on upgrading to Golden Gate. I was hoping there might be a .conf file or other I could edit to get rid of the upgrade detection so the notification will not be in the System Setting.app or the notification badge.

It is quite disappointing that there is no way to get rid of this stuff if a user doesn't want it.