Is it still considered a runtime dependency if all you do is merge code and deploy while relying on GitHub? I assume most apps will not go down if GitHub goes down, but their ability to move code to production gets stalled.
The beauty of Git's design, though, is that you've still got everything to use it distributed even if you, under normal circumstances, just use it centralized like a fancy SVN.
There are probably a lot of other tempting features to put on your deployment critical path but I'm not in an environment that uses github so I've forgotten.
It's not a runtime dependency, it's a stupidity to rely on random repositories from the internets when building and deploying (remember the left-pad farca that halted work for half of the web programmers). You should always have your local copy of necessary sources or use repositories that can trivially be swapped with some other mirror.
We used to have the infrastructure for those things (in the form of binary and source package repositories), but it was mainly used by sysadmins, and programmers usually have an alergy for such infrastructure.