Currently on latest Catalina beta build. The command does nothing for me. Doesn't throw an error, but no address change either. Might add I have 2019 MBP.
There’s isn’t one. It’s just “Coming in October”. But, the GM build was recently pushed to beta, so probably pretty soon.
It's generally about a week after the GM release, so you should expect it pretty soon. It was released last friday, so it might be end of this week early next week.
Note that you will still see the original hardware address in System Prefs > Network > Wi-Fi > Advanced, but ifconfig shows the changed MAC, which is indeed the one that is broadcasted