I’ve recently installed Steam on my Mac for the first time. I made sure to disable the auto startup option from the menu as I hardly use it on this underpowered machine (under Steam – Preferences – Interface). There’s no tick box there!
Imagine my surprise when I restarted my Mac a couple of days later, only to be greeted by the familiar window showing “Updating Steam”. Bad software design at its best: do the opposite of what the user has selected. Good job, Steam!
This being a Mac issue, there’s about 0 threads on the web on how to tackle this issue. It took me a moment to remember that there’s built-in Mac option under Preferences – Users – Login Items. That’s where Steam was hiding, and as such, it is started no matter what. Untick that box, and Steam will no longer start on your Mac when you boot the system.