I have looked into what you can do to avoid this some time ago since as sysadmin for my wife I would make sure she did not upgrade her mac while I was out fishing and found this some months ago https://forums.macrumors.com/threads/how-to-remove-ventura-upgrade-notification-forever.2367582/post-31689711
It will, as I understand it, have to be reinstalled every 90 days. It has worked well for 2 months and I have set a reminder to reinstall 01/02/2024 on my wife’s mac.
I have made a checklist based on my profile installation.
Download iMazing Profile Editor from AppStore
Ignore all menu items I do not mention!
Install the app.
Select “Manifest Repository Updates” from the main menu.
Reload Profiles.
Restart the app.
In “General”, give a name to the profile (Defer-MajorUpdates)
Set “Removal date” Some date in the future.
In “Payload scope” select “System”
In “Target Device Type” select “Mac”
Then in the left vertical menu, select “Restrictions”
Click “+ Add Configuration Payload”
Make the window wider so you see the menu across the top.
Select “Updates”
Select “Defer Major OS Software Updates “
Add 90 to “Deferred Major Software Updates Delay”
File>Save
File>Install
You will be prompted to allow it. Search for “Profiles” in “System Settings”
After a reboot, it worked.
It is easy to remove.
Edit 2024.01.16. Added missing “Deferred Major Software Updates Delay” the default is 30.