It is likely that John Cantlie is dead. John was last seen in a video released by the Islamic State in December 2016 called Tank Hunters.
It has been 3 months since the battle of Mosul was declared over. Tal Afar was liberated shortly after. John was trapped in that pocket. Since the battle is over, we have not seen John alive.
According to interviews with captured Islamic State fighters, John was killed in the battle. It is unknown if this was from the battle or was executed. Above is a compilation of the last video he was seen in.