That's a bit of an absolute statement! I should imagine there's still unknown stuff about.
It's just that we don't know where it is!
Big freshwater lakes are still yielding fairly intact airframes. Russia is still a very big place with a lot of forest. And there is probably the odd person with secrets in the back barn.
But the "buried intact airframes" rumours have run and run and to the best of my knowledge have all been unfulfilled - which is a pity.
