alt Hacker NewsGithub used to be built on mysql / redis / ruby on rails / C / shell, running on dedicated hardware. Microsoft left it like that after they acquired the company.
Eventually, though, they decided to migrate the whole thing to Azure. And they were far enough through that to be basically committed... when AI coding started hitting them with much higher workloads.
I personally think the reliability problems are more to do with the reliability of the Azure migration. But both factors are likely relevant.
After their infamous "Fail Whale" outages, Twitter replaced its Ruby on Rails / MySQL stack with modern (at the time) technologies: Scala (functional language running on the JVM) / Finagle (RPC) / Thrift (framework) / Kafka (message queues) and this made Twitter stable under the load of several hundred million users.
Ruby is single-threaded, slow with poor GC and memory leaks. As Twitter's Alex Payne said publicly in 2008: "there shouldn't be doubt in anybody's mind that Ruby itself is slow" and that scaling Rails meant "just throw more instances at it"