The Valeyard's fate was resolved in Big Finish's interqual to the Sixth Doctor's regeneration into the Seventh Story. Though we have a copy of the Valeyard out there who thinks he's the Doctor (also a Big Finish Story, "The War Valeyard").
Moffat is the only one who respects Big Finish it seems. But the only thing that would keep RTD from using the Valeyard is it would contradict the Timeless Child.