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.