If GitHub is offline we can still setup a git server somewhere. I could offer my own for a quick startup. Mailing patches to each other, Linux kernel style, is not a viable backup plan. The cultural gap is too wide.
If Travis is down we can run tests locally.
We build the deployment artifact on one of our servers. If that one is down probably our production server is down too.
If Slack is down, ah, I was on vacation yesterday. I guess the fastest backup for us would be WhatsApp Web.