I would be extremely surprised if they killed Carl off, but at the same time, I'm not sure what other kind of plot development could explain his absence. Maybe he goes away on a journey - ala Morgan - and we only see him once a season until Chandler is finished with college? Maybe a massive time jump that would require a much older actor?
I won't be surprised if Heath dies in tomorrow's episode, however. With Corey Hawkins playing the lead in 24: Legacy, I doubt he'll have time for the show anymore.