How to stop Mac OS from checking for and asking to install updates?

I am trying to stop my MacOS from periodically checking for updates and displaying these notifications:

I have unchecked seemingly all relevant checkboxes in App Store preferences:

And in System Preferences / Software Update as well:

Yet every time I open the latter it defiantly checks for updates:

How do I get it obey my directions? I am aware that some updates are required to actually fix security issues (as opposed to enhance the telemetry / data collection / spying etc. for the benefit of Apple). Yet I expect the OS to do what it apparently supposed to: not to check for updates if the checkboxes are unchecked (otherwise why would those checkboxes exist in the first place?). I am happy to take the risk to live without updates where I choose to, and I do not need Apple to override my decision.

This is Big Sur 11.3.1 (20E241).

Solution 1:

• The App Store preference only controls the automatic update, not the checking whether an update is available. The later happens automatically
• Software Update checks when you open the panel, independent of the "automatic" setting. If it detects an update it shows it in the menu

So basically it works in the way defined in the preferences.

PS: Some of the answers on How can I permanently disable App Store notifications in Big Sur? might be useful

Solution 2:

If you wanted to prevent the OS from checking the software update catalog, you could try changing the catalog's URL:

sudo defaults write /Library/Preferences/com.apple.SoftwareUpdate CatalogURL http://www.example.com

... and then when you have somehow satisfied yourself that updates are ok, you can manually check for them, by removing the preference key:

sudo defaults delete /Library/Preferences/com.apple.SoftwareUpdate CatalogURL 

Note that the OS will not be able to find any system updates until the catalog is restored.
(You may want to keep a copy of the unmodified /Library/Preferences/com.apple.SoftwareUpdate.plist, just in case.)

Alternatively, you could just auto-install and roll back if concerns are discovered about an update.