Small to mid sized OSS projects benefit heavily from this. There is a size beyond which the free runner sizes become insufficient, but the assumption is that some form of monetization is figured out by that time. For example, we have a lot of OSS projects using WarpBuild because performance and fast CI is important for productivity.

Without GitHub's free CI for public repos, the small projects and indies will get hit the hardest imo.

However, I do not know hard numbers to quantify the impact.