They seemed to be suffering some external attacks / DDoS but I never saw a post-mortem from them on it, hopefully one is forthcoming (or maybe is out but I missed it)

They occasionally do, and they call them Incident Reports.

https://github.com/blog/2106-january-28th-incident-report

https://github.com/blog/2273-incident-report-inadvertent-pri...

Maybe not public ones, but I'm pretty sure they perform internal post mortems when they have outages.