Because someone’s OKR or some other metric depends on it and they will keep it from being on the status page as long as possible until there’s no denying.

I just set-up an automatic status page tracking Slack, in case you're looking for confirmation in the future: https://slack.onlineornot.com/

(no incidents listed atm because I set it up after they came back online)

Uses an uptime check against their API and website to independently check that they're still up.