Card effects don't do anything except what they explicitly say they do. Slumber does not remove or suppress traits. Drakas Imp Overlord would still have those abilities in effect.
Similarly, if he was removed from the Arena, such as by Banish, that also does not remove or suppress any of his traits. However, since his traits only affect demons in his zone, and he wouldn't be in any zone, they would in practice be useless, but they're still technically active.
