IIRC, the devs updated their answer since they have done way more testing on old worlds in the meantime.

Here's the info from the pinned FAQ post:

"Will new content be added to my world I currently are playing on?

New content such as tar pits, mountain caves and Mistlands in the future will not appear in areas that have been already generated. Areas are generated 300-500 meters from a player's location. "Exploremap" and "resetmap" has no effect."