I think this is being downvoted unfairly. I mean, sure, as a company accepting payment for services, being down for a few hours every few months is notably bad by modern standards.
But the inward-looking point is correct: git itself is a distributed technology, and development using it is distributed and almost always latency-tolerant. To the extent that github's customers have processes that are dependent on services like bug tracking and reporting and CI to keep their teams productive, that's a bug with the customer's processes. It doesn't have to be that way and we as a community can recognize that even if the service provider kinda sucks.
Being snapshot-based. Git has some issues being distributed in practice since the patch order matter which means you basically need to have some centralized authoritative server in most cases with more than 2 folks to resolve the order of patches for meaningful uses as the hash is used in so many contexts.
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.