I found it much worse that installing anything STILL by defaults upgrades literally everything. You have to actually set an ENV variable to stop this behaviour. My stuff constantly broke because I'd forget about this and install something else and a couple days later I'd go "what happened??"

I don't think brew does this? What it does do is update its package definitions automatically.

It was doing this until recently when I set the env variable. It updates itself every time and then updates everything else too. Maybe I did something wrong to trigger this behaviour but it was definitely updating the packages

I've seen the exact same behavior. I didn't realize it was behind an environment variable. I passed a package to the command and for the next week I was dealing with issues from everything being upgraded.

Why would you run brew upgrade if you don't want to upgrade packages? I don't understand.

You'd expect MySQL 5.6.8 to upgrade to 5.6.12, not to 5.7...