alt Hacker NewsI don't think GitHub is quite as important as that, outside of some specific projects that made bad/lazy decisions (thinking Golang here). If they didn't support 256 I think a lot of enterprises and open source stuff would simply jump ship to GitLab or elsewhere. In the enterprise world it only takes one person to write a security document banning sha1 for this to happen. For that reason, GitHub will support sha256.
I think you would be very unpleasantly surprised to see how many organizations deeply rely on GitHub. Think about how many open-source projects you've seen that have willingly tied themselves to GH-specific features, and then extrapolate to thousands of companies with masses of spaghetti code that are hopelessly reliant on their internal GH Enterprise features.