It's actually unknown.
The last time we see of him is Biscuit Oliva cornering him in the sewers after he was blinded and fatally poisoned by Yanagi. We later find out the only way to cure Yanagi's poison is by a martial artist who uses a similar but reverse technique. The only one we meet in story is at Kaioh tournament which Oliva escorts now infantile Dorian Kaioh too, becaise even though he is a mentally impaired prisoner he was still a Kaioh.
Theoretically the author could say that Oliva brought Doyle the Cure in prison and bring him back later in the story.
If you are thinking, but he would still be on death row, please remember Doyle is Scottish. When this part of Baki was being written the UK still had death sentences for war crimes and treason, but those were removed so the author could say that the death penalty outlawing saved Doyle.
It's unlikely he will come back, but it's not guaranteed that he is dead.