I'd guess most "new sections" are just part of the Mines/Catacombs of Paris whose access was previously blocked by the official organisation in charge of them, the Inspection générale des carrières.
Maybe. I definitely saw people digging though, and I went through large sections that were newly dug. This was in 2014. Also, sometimes new building foundations are cut into the existing tunnels, and new tunnels are also dug into building foundations. Interestingly, the cataphiles I spoke to strongly 'discouraged' theives from entering buildings in this way. Something about not drawing undue attention from authorities.
I heard that story of authorities blocking some tunnels and cataphiles removing the blocks from someone who went down there; I don't know how widespread it was.
It's a common game of cat and mice, still "played" to this day. When they can be bothered enough, authorities will fill a section of a gallery with concrete. And, in turn, when cataphiles can be bothered enough, they tear down a piece of an adjacent wall and dig through the dirt on the other side to go around the blocked section. I've also seen crawl holes dug under 1m-or-2m-thick rubble walls.