There are still some processes that require a waterfall method for development, though. One example would be if you have a designer, and also have a front-end developer that is waiting for a feature to be complete to come in and start their development. I know on HN it's common for people to be full-stack developers, or for front-end developers to be able to work with a mockup and write the code before a designer gets involved, but there are plenty of companies that don't work that way. Even if a company is working in an agile manner, there still may come a time where work stalls until some part of a system is finished by another team/team-member, especially in a monorepo. Of course they could change the organization of their project, but the time suck of doing that (like going with microservices) is probably going to waste quite a bit more time than how often GitHub is down.
> There are still some processes that require a waterfall method for development, though
Not on the 2-4 hour latency scale of a GitHub outage though. I mean, sure, if you have a process that requires the engineering talent to work completely independently on day-plus timescales and/or do all their coordination offline, then you're going to have a ton of trouble staffing[1] that team.
But if your folks can't handle talking with the designers over chat or whatnot to backfill the loss of the issue tracker for an afternoon, then that's on you.
[1] It can obviously be done! But it's isomorphic to "put together a Linux-style development culture", very non-trivial.