Approval of God: Maddy is a supporter of fangames, mods and ROM hacks, and has given permission to a crossover called Celeste.smc that has Mario explore a similar mountain to the one in Celeste.
The code contains a "golden block" which rushes up from the bottom of the screen if the player enters its room while carrying a Golden Strawberry. The obvious intent is to make the player take an even harder path on an already-difficult challenge, but the game as released accomplishes this by adding an entire extra room with established blocks, and these special blocks go unused.
The "nega block" is very unfinished (its graphic is literally solid red with no texture). If implemented, it would work like a normal block except that it doesn't refresh Madeline's dash when standing on it.
Limited Special Collector's Ultimate Edition: The game received a physical Collector's Edition for the Switch and PS4 versions, courtesy of Limited Run Games. Along with the game, it includes a steelbook case, the soundtrack on a cassette tape, a patch, a postcard, a poster, a sticker sheet, a stuffed winged strawberry, and a strawberry pie recipe.
An old development outline for the game shows that 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 (the conversation Madeline has with the old woman in Chapter 4 is a remnant of this).
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.
There used to be a mechanic that let you dash forever in the air, but it was scrapped because it "wasn't really that fun".
Word of God: 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.
Write What You Know: According to Maddy Thorson, much of the game (especially Madeline's portrayal) was designed as an allegory for her experiences coming to terms with her gender identity.