No it's not. Just asked my friends in China and tested with https://www.comparitech.com/privacy-security-tools/blockedin... (try with github.com and reddit.com). The Chinese are heavily invested in GitHub, so if/when they do ban it then you can be sure it won't go unnoticed.
GitHub has been banned in the past and yes it didn't go unnoticed ( https://news.ycombinator.com/item?id=5090700 ), but that's not much consolation when you suddenly can't access it. Currently they mostly don't block it ( https://en.greatfire.org/https/github.com ), but if GitHub ever became popular among ordinary citizens (as opposed to programmers) downloading e.g. censorship circumvention tools from GitHub, they'd probably find a way to mirror most git repos, except those they want to block.
Obviously there is a limit to what GitHub would be able to get away with before getting banned..
My point is that GitHub currently provide so much value to Chinese research and tech that banning it would be extremely costly for them. Would they ban CNN for writing a negative article that not a single Chinese citizen had seen? No doubt. Would they ban GitHub for hosting a repo sharing the truth about the Tiananmem Massacre, the concentration camps, etc? No, at least not until this repo gained a lot of traction in China, and the question then is how many people would they allow to see the content before finally pulling the trigger? I'm imagining a lot.. And I also suspect that they would respond by attacking GitHub/Microsoft and even open up for GitHub again when they felt the storm had been weathered. FYI, all of these assumptions are based on their past actions with regard to GitHub and other websites/companies.. if you read up on it then you would probably reach the same conclusion.