Celeste was designed to be a challenging, but accessible game. We believe that its difficulty is essential to the experience. We recommend playing without Assist Mode your first time. However, we understand that every player is different.
From my perspective as the game's designer, Assist Mode breaks the game. I spent many hours fine-tuning the difficulty of Celeste, so it's easy for me to feel precious about my designs. But ultimately, we want to empower the player and give them a good experience, and sometimes that means letting go.
The goal is a fluid experience where players are safe to float around between loosely-defined difficulty levels as suits them, without judgement or implication that they aren't playing the game ‘as intended.’ Hopefully players feel safe enough to experiment, connect with the story, and express themselves through the game systems.
Why The Very Hard 'Celeste' is Perfectly Fine With You Breaking Its Rules - Waypoint