Yarnaby's original death:

Originally, Yarnaby's boss fight and death were a lot different. We were going to fight him in the upper levels of the factory, where The Doctor had sent us. The battle was set in an area filled with machinery and conveyor belts, the very place where the toys were made. Instead of dying by falling into molten lava, the machines in the area were going to kill Yarnaby by tearing off his yarn and limbs, dismantling him.

This was said by the former director (Isaacx) in a recent interview who wrote the story of Chapter 4 before his departure.

We were so fucking robbed...