Destruction of the worlds of the Graveyard was so thorough that not a single bit of organic matter remained. Because of that very little is known about them, other that the simple fact that they existed (and were pretty mean, not to mention having technological prowess required to explode stars).
It is even impossible to learn when exactly the destruction happened and thus when Destroyers existed. There are many theories about that, made mostly on a basis of at least partially inconclusive scientific researches and ranges from a million years ago to merely a hundred years before the sector was discovered.
They are believed to be extinct - due to absolutely no signs of their activity - but there is at least a single haunting theory that they simply passed through the area on their devastating crusade, but without an access to Hyperspace Drive
, making them slowly expand via either another FTL Drive or by STL speed. There is no need to return to already cleansed systems, but the Destroyers themselves might still be around, simply in the vast void between sectors.