In fact, there's an inverse incentive at play here. Slack is on the hook for missed SLA, so any admission of issue is legally and financially culpable. This means they don't want to admit to an issue until it's impossible to deny there is one.
We recently improved our status reporting because our old status system might not accurately indicate if only a part of our service went down, so it might show green even though (some) customers definitely saw very obvious downtime.
Wait what? And SO agreed? Where can I learn about this?
In those cases status pages are more likely to have a lovely spectrum of yellows and oranges to choose from, but a distinct absence of red, but very few companies will say that they're fully green when they know there's issues. Green when you're down is usually more of a symptom of process problems in how outages get reported or resolved (no documentation, manual processes for updating, long communication chains from support teams to engineers).
How many customers will think you look dumb? If you admit downtime, will the accurate reporting drive away more new customers, than the inaccurate reporting drives away due to you looking dumb?
"Looking dumb" is not a reasonable factor. Damage to your public image is, but that's divorced from looking dumb. It's entirely possible Slack is saving money by not admitting their downtimes, because it leaves a better image.
Slack now has the outage listed:
https://status.slack.com/calendar
The online cynicism is something I've never understood. It assumes the world is filled first first-order simpletons & somehow only the poster rises above it.
People aren't dumb & companies aren't dumb. People will notice a status tracker isn't accurate. They will have reduced confidence in the company. Companies know people will notice & think this way, so they generally don't engage in this type of deception. I'm sure there are exceptions, but in general, status pages have tended to be accurate in my experience.