Isn't there the same chance that any domain has that chance? I'm struggling to see how Slackstatus.com is any safer than status.slack.com. Genuinely curious - I'm not trying to start an argument:)
A change that is capable of bringing down slack.com will probably take down status.slack.com too. Whereas slackstatus.com would (hopefully) be more isolated from such failures.
It's true that your overall chance of failure is now higher, what you really don't want is for both to be down at the same time.
Not necessary true. As long as status.slack.com is it's own separate A record, as long as you don't change the DNS Zone for slack.com, they are pretty much separate. Now, if they use a WildCard SSL for *slack.com and that goes down, then yeah. But other than that, having it on a sub domain shouldn't be a major issue. Also, I'd much more trust the status.slack.com subdomain than slackstatus.com, which might be some 3rd party thing.
It's true that your overall chance of failure is now higher, what you really don't want is for both to be down at the same time.