VMS clusters were (and modern equivalents are) awesome, but they have a different speed of innovation / development.
It is a trade off - perfect uptime, but that new feature could take a year, or 99.9% uptime, but it can be written and deployed tomorrow.
https://www.youtube.com/watch?v=4fFDFbi3toc
Generally, on top of code reviews, one can get high return with minimal labor and hardware with a combo of Design-by-Contract, contract/spec/property-based testing, low-F.P. static analysis, and fuzzing with contracts in as runtime checks. That's my default recommendation.
In Github's case, they also might have access to both closed and open tools that Microsoft Research makes. MSR is awesome if you've never checked them out. Two examples applicable to system reliability and security:
https://www.microsoft.com/en-us/research/project/ironclad/
https://www.microsoft.com/en-us/research/publication/p-safe-...
Plus some of their other tools in various states of usability:
https://support.hpe.com/hpsc/doc/public/display?docId=emr_na...
These modern cloud services using Linux ecosystem are doing rolling outages instead. I'm amazed people keep making them dependencies. Maybe their local systems using similar technology went down more often. Maybe this is an improvement. I'd still put mine on an OpenBSD or OpenVMS cluster if it was business-critical with lots of money on the line. I want it staying up, up, up. :)