I've been tearing my hair out trying to get rid of the quarantine "Do you want to open this?" warning because I don't like it.
The well documented way (defaults write com.apple.LaunchServices LSQuarantine -bool NO) does NOT work.
Also, I've used the Secrets Preference pane and that doesn't work either. As soon as I uncheck the box and close the pane and reopen, it's still checked.
(I've tried rebooting right after these methods, no dice.)
Best Answer
Never tried to disable the behavior entirely, but you can always remove the quarantine from a file with xattr. For example:
I think of it less as a moron check than it is a safeguard from browser vulnerabilities that might otherwise download and something without your knowledge.