I made contributions to Supabase before joining the company, although my contributions weren't to any of their GitHub codebases but more of a community one where I answered questions on their GitHub discussion board on a regular basis for users who were less experienced with the platform than I was.
I am not sure if you are referring to the same sort of filtering mentioned in the document. The data is filtered before it hits the database layer, this saves resource being used.
There is a bit in the Introduction section of the doc that says how to contribute. Its actually just a one page on github at the moment that you can fork and send in pull request.