New content will appear on old worlds but only at locations that were not generated/saved before the patch.

Some enemies are an exception however. For example, abominations will spawn anywhere in any swamp regardless of which patch that piece of world was first generated in, because abominations are not tied to a specific structure.
But enemies like growths rely on tar pits in order to spawn and therefore cannot be found in places that were generated previous to the implementation of tar pits.