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None of that stuff is exclusive to github.

The would work exactly how they work with the Linux kernel, for which git was created.

Yes, via centralized mailing lists and collaboration infrastructure.

... None of that stuff requires github.

How about also have the code on bitbucket (if you want to use flashy web services). Chances of them going down at the same time seems remote.

Actually its more likely that they're both unavailable than one is I.e our pipe goes down. Another reason to do it in house.