I doubt moderators would allow to choose nick "Deleted member" so I think they are really deleted. (I hope everybody remember Lea the Troll or Wassuplol)
You should have noticed option to delete your account in settings.
The majority of times "Deleted Member" shows up as a name is when that profile was deleted due to spam issues. Eventually all the messages get purged but there are times when "Deleted Member" appears as a placeholder. That's pretty common in online fourms.