This is actually impressive, in a bad way. I just have become so used to being able to run highly resilient cross region infrastructure for millions of users with just a handful of people that I forget what real downtime looks like. For their app to just go completely offline is unacceptable. Bugs and degraded services I get. But this is catastrophic.
I can't even begin to guess what went wrong. What are your guesses? How many screaming executives are there at Slack saying "just roll it back"?
Doubtful it's a code issue causing a total system outage. I'm assuming they have a bunch of auto scaling infrastructure that wound down over the holidays and couldn't take the spike this morning.