The monsters that chase and attack Madeline in Chapter 5 are unnamed in the game, but were officially named "Seekers" during a speedrun of the game at SGDQ 2018 that was commented on by the development team.
Chapter 6 was originally going to be a water level, but it didn't work out.
Chapter 8 was originally going to be set in space, as opposed to in the core of the mountain, and that there would have been a Hot Springs Episode between Chapters 3 and 4.
According to Maddy Thorson, had she known that she was trans before starting development on Celeste, she would've depicted Madeline (whose identity as a trans woman is only alluded to in the final game) much differently, though didn't give any specifics beyond this.