He's done, there were several articles from Seminole Warriors Boxing and other sources and even an interview with Sultan, as recently as Feb or March this year, confirming he's retired. He was looking at retirement even before the Wlad fight. The guy's a millionaire and moved back to Russia and apparently has a decent life there with his family. I wouldn't expect to see him again in the ring.
For some reasom I've always liked the guy. He was clever and effective with that sneaky defensive style of his and always, until Wlad, that is, managed to what it took to win, as in his title winning fight with Briggs. He made the most of what he had, and fought well and effectively within his limitations.