So everything's normal for me. If this were happening to everyone it would be widespread and huge news.
So assuming it's some kind of possibly-obscure bug, if it's only a handful of people?
1. Automatic updates set to "on" (because automatic updates only installs point releases, not major revisions).
2. "Updates are going to be installed tonight. Install now?", or words to that affect. That's the usual message for point release/bug fix releases. Oddly, when I click "Details", there is no point release and just the Sonoma upgrade. This seems to be a bug, IMO. The "upgrade to $LATEST!" should be a different dialog with different wording.
3. System Settings still shows "software update available", with the red "1" badge, when Sonoma is the only update. This seems to be a bug as well.
IMO, some state-dependent $THING got hosed, and now things are just a little off on my machine. this manifests as an unwanted OS upgrade for some, and just general "that's not...the way it used to work" others like myself but no forced upgrade.
It's not only a handful of people. There are already a handful of people verifying the issue in these very comments here, in addition to the people who have verified it in various other linked forums.
Now I’m worried that this may pop up again and perhaps download and install the upgrade sneakily. I’m going through online posts and comments here to see how I can prevent it.
Apple as far as I know has always respected your update settings, so at this point I don't think it's intentional. But obviously this shouldn't be happening.
I run Sonoma on my Official Macs, but I also have a Hackintosh on Ventura that has not upgraded.
It has an option to Install macOS updates automatically.
Another option could be about "Security Responses and system files". Maybe delayed a security patch for Ventura for a long time, so macOS decides to install Sonoma with the last update?