Isha and Cegorach are still around, albeit somewhat inaccessible. One might also count Khaine as being around collectively, as it were, and I'm reasonably sure there was a reference in some Codex Eldar or other that some Old Ones may have fled into the webway and sealed it off to prevent pursuit.
Having been proverbially sharpening their axes for the last 65 million years, it wouldn't be too hard to imagine that the Necrons are so petty as to still be hung up on a war long over and consider any remnants of the Old Ones as being enough reason to burn worlds again.