There were additive approved by EPA as not poisoning the emissions systems
Much like having lead in your new vehicle, it would kill the catalytic (story of out of fuel on a cycle and the guy was worried about me putting aviation leaded fuel in it!)
So, no VW does not approve additives. Of course they don't need it.
But my DOC apparently got a bit too much 500 ppm diesel (Ak had an exemption to that level) and smokes badly (ergo Smokerr moniker) if I don't put in a dose of additive every 4 ort 5 tanks now)
I could never prove it was smoking, take it into dealer and nothing. On the way home clouds of smoke.
Then my wife tells me, it stopped smoking and it always smokes on that steep hill up to So and so. Hmmm.
So yes it works, 102k now, doing fine and the additive obviously does something.
But it also is a specific additive for diesel by an injection pump mfg (no longer made but I have a stock pile)
Purely found out by accident as I wanted to ensure I had enough lubricity in the then new ULSD in winter.
Other additives out there and our fuel supplier for facility engines actually adds it into their diesel automatically now. They wanted insurance as well.