So, while I was looking for ways to stop those auto-updates.. (trivia: there was never a setting, as explained in my new reference)
..I stumbled upon this. What do we now about Steam console?
I tried the method suggested there, but it didn't work.. Though I had never heard of this steam://open/console, seems interesting
btw, I found "Automatic updates" setting is no longer saved where claimed in the previous link, but instead in each game appmanifest_####.acf