When you say "on by default" makes me think that even if
define( 'WP_AUTO_UPDATE_CORE', 'minor' );
isn't in my wp-config.php file, it'll still update automagically. Correct?

Or do you just mean that in new installations that line of code will be in new config.php files?

I just updated one of my sites and see that like of code is not in the wp-config.php file.

Thanks!
Gary