Why is the System Preferences lock decorative and no longer preventing changes when locked?
On Mojave, the categories with no padlock requirement are Contacts, Calendars, (no app has requested access to Reminders yet), Photos, Camera, Microphone, Automation and Advertising. 

Those requiring Admin password are Location, Accessibility and Full Disk Access.

Analytics does not require unlocking to change Share iCloud Analytics but does for Share Mac Analytics and Share with App Developers which are otherwise input inhibited. 



Former list permissions didn't require me to enter root password as far as I remember. So it shouldn't ask on a later time either. 

You can check whether password is required or not by 

tccutil reset Microphone


or any other item like AddressBook. See this blog for the same

https://www.felix-schwarz.org/blog/2018/08/new-apple-event-apis-in-macos-mojave or man tccutil. 

As for those protected ones, +- buttons are given and they are greyed out if padlock is locked.