Is this on by default?

IIRC yes, unless your internet connection is marked as Metered, in which case the default is off.

What I can't remember and am unable to check at the moment is whether Metered is turned on by default or not.

But regardless of defaults, if you mark a connection as Metered than the background update distribution is turned off.

Metered is part of your connection settings, iirc, like when you establish a connection to a new wifi network. Other applications may be effected by metered networks, and iirc updates don't run at all when on a metered network.