It's too bad git requires a central server.
Github's not only Git. Lots of services integrate with Github. Stuff like continuous integration, continuous deployment. If your system is built on those things, Github being down will prevent you from deploying.
What's the alternative, replicate everything in-house/self hosted? Should startup stop using third party service providers?
I continue to post comments like that because while I love GitHub, watching the community put all their eggs in GitHub's basket, especially when GitHub lives on top of a tool designed to avoid SPoFs, is concerning.
While I agree with the general gist of your comment, GitHub was entirely self-funded up until about a year ago or so. they didn't "get" funding, they made real money and reinvested it and grew.
You're saying it as if it were a bad thing.
Erm... Yes? You write that as if hosting your own repository is somehow a foolish or implausible thing to do.
Should startup stop using third party service providers?
Stop? Maybe, maybe not, it obviously depends on the circumstances.
Stop relying on them without a back-up plan? Absolutely. If you can't run your essential systems without GitHub, or any other third party system that isn't under your direct control, then you need better contingency planning.