At the moment the only way to tell if a player has disconnected is to look in the console logs. This is irritating during gameplay, as it takes up time and can be knocked off by more recent console messages. In addition, it can also cause gameplay related problems when a traitor disconnects, because no-one knows how many are dead. My suggested solution is adding a new section to the scoreboard for disconnected players, essentially a "confirmed dead" list for players who have left the game. This would fix various gameplay problems involving bad communication and misunderstandings involving the traitor count.
I'm not sure if this is possible technically, but I'd assume so considering the scoreboard is custom. Tell me otherwise if I'm wrong.
Here is a mockup of this idea: