[cywick]

Have you tried rebooting your Mac? Because on both of my Macs (a MBP 2020 and a Mac mini 2018, both running Ventura), the involuntary Sonoma upgrade only started once I manually rebooted the machine. Before the reboot, everything seemed normal and there was nothing indicating that the upgrade would take place.

After experiencing the issue on my MBP, I actually waited a couple days before I rebooted my Mac mini, and before I did so, I checked whether there was an "Install macOS Sonoma" app in the Applications folder (there was not). My (now proven incorrect) assumption was that the Sonoma installer would always come in form of such an app, like it did for previous versions of macOS.

For the past 5+ years, I've always waited at least 6-9 months before upgrading to a new major macOS version, and I am generally very careful about not upgrading accidentally. There is zero chance that I twice missed a notification or accidentally initiated the Sonoma upgrade myself.