Has it? We don't use GitHub in my company, but our self hosted Gitlab occasionally goes down, and while it prevents us from merging (including doing code reviews) and deploying code, it does not prevent us from doing most of our work (ie designing and creating software). It merely delays the moment when things are shipped.

If you meant goes down for good, then I'm sure it would be annoying for a few weeks for the FOSS ecosystem, just the time to migrate elsewhere, but there is not much GitHub specific we would really miss.