These chunks will never get deleted. They have more than 20 minutes of in game time.
It also means they’ve locked in whatever terrain generation that was around during that version.
For example, you can find chunks in Melon Falls that don’t have diorite, granite or andesite.
Areas on the borders of permanent chunks may have also include weird terrain changes, these pre-date the game being able to seamlessly merge old chunks into new chunks.