Someone who needs to clone your repository (e.g. you're a dependency for their project) cares.
Someone tired of using an inactive repo and trying to get an overview of healthy forks cares.
Someone whose project uses GitHub for more than just a raw git repo (e.g. issues, wiki, releases with precompiled/crossplatform binaries) cares.
Someone searching for answers who encounters a link to some GitHub gist cares.
Not if youre Joe Chinese Dev wanting to collaborate with an OSS project hosted on Github. "Hey, can you guys move everything to this service that doesn't block China so I can send you push requests/issues/etc" is probably going to go nowhere.
They can use git. They can use mailing lists. They can use carrier pigeons to exchange pull requests. That's not what the concern is about.
If the OSS project says that all collaboration work has to happen on their Github projects, and Github's forced to block China - then Chinese devs would no longer able to collaborate.
They would need someone outside China to be willing to take their pull request/log an issue/etc.
My point is to underscore the point of the ancestor of this thread: that you (the project) don't need github. You can use all the strength of git without github.
I was doing some analytics on various crypto projects on GitHub, and was surprised by the number of them that had authors without GitHub accounts. These show up in the commit API as having 'author' and/or 'committer' as null (the commit itself has an author that's whatever you set your "git config user.name/email", which may not be a registered GitHub user). You have to handle these cases or your analytics script will crash very soon, because it's a pretty common case.