I know that in King's Quest 5, if you go into the dark forest without the honeycomb or the bottle, you can get eaten by one of the plants or by a spider. It seems like this only happens if you've trapped yourself in the dark forest by not having everything you need.

I also learned that Sam can appear just outside of Mordack's castle if you didn't pick up the crowbar or the crystal.

Are there any other deaths that only happen if you made the game unwinnable?

There are puzzles that you can solve incorrectly that makes the game unwinnable and could lead to some deaths such as giving the Eagle the Custard pie instead of the remaining leg of lamb which means death over and over again by the yeti. Also not helping the Eagle can lead to death by the two headed bird where you get the golden locket from which can make the game unwinnable if you saved your game along the way without any backup saves.

Also throwing the rope up to the branch and saving your game afterwards with no backup saves can lead to an unwinnable state which can can lead to death by attempting to climb.

I also learned about the Death by Sam out side of Mordack's castle.

Also there are alternate ways to solve puzzles like the golden needle to get into see the fortune teller and the gold coin to get the cloak. You can make the game unwinnable by giving the golden heart to the clerk who sells custard pies and the same goes for the gold coin if you all ready given the golden needle to the tailor for the cloak.

Naturally, leaving Cedric behind on Harpy Island will get you blasted by Mordack in the endgame.

Oh Yeah. I forgot about that one.