Forced upgrade to Sonoma: can it be stopped?

Numerous users are reporting that their computers have been forcibly "upgraded" to Sonoma without their consent. It seems to have started on my Monterey boot drive today: I am currently booted from another drive and am afraid to go back to using my main system drive (Monterey). I ALWAYS have Auto updates turned off, with only "Check for updates" and "Install security responses and system files" enabled. Is there a fix for this? I don't want to have to spend a chunk of time getting rid of Sonoma and restoring Monterey, and it's ridiculous that anyone should have to.

TidBits Thread on forced "upgrades"