Turning on the developer options menu has been the same for who knows how long. You always open settings, go to 'About phone,' and tap on the build number for a certain number of times (though most of us simply tap on it furiously). However, the second Android O developer preview has added another step: entering your device's PIN, password, or pattern.
As you might have been able to guess, this extra step is for security purposes. After all, you wouldn't want someone to unlock your bootloader or enable USB debugging without your knowledge. For instance, unlocking the bootloader disables factory reset protection, which means that a thief could get away with a stolen phone that was unlocked when taken just fine.