You can climb walls with the right trigger. A lot of people haven't realized this, seems like a conveyance issue.
After navigating the comments I found the wall jump trick: it is naturally hard due to the gameplay style, but the problem is that it is not conveniently explained in-game, specially for new players. A lot of people who has never played Celeste before (myself included) found attractive a N64 inspired game from an outstanding indie title, so a simple tutorial may be good.
Even tho, for an early release of the game it is technically completed, I haven't found bugs at all.