The game accessibility guidelines is a collaborative effort between a group of studios, academics and specialists who released a living document used by creators as reference for implementing accessible features to their projects.
Even the inclusion of an assist mode similar to Celeste's and could prove useful for developers seeking to make their games more accessible for a wider range of players.
As well as including a full list of considerations (which contain the basic/intermediate/advanced guides), each guideline explores a facet of the most commonly requested accessibility features. For example, the basic guideline includes how to provide subtitles for important speech for any players who might be hard of hearing.
More information about how the resources can be used are listed on the game accessibility guideline's website here.