In a typical work environment, I also agree. But somehow, Slack is also a popular choice for community hubs of open source projects.

I've seen a couple of those moving on to Discord, I guess it could be related to the lack of "social" features like this.

Exactly. And the right solution in this case would have been to contact HR and file a harassment complaint. (Or alternatively, tell the harasser that if they don't stop immediately, HR will be notified.)

If Slack is being used as an office communication system, the company needs to make sure that people treat it as part of their office (like corporate e-mail), not as some kind of social network.

In practice how this is works is that the person being harassed goes to HR, HR does nothing, and then that person quits. Nothing usually happens to harasser. The point of HR is to protect the company from its employees, not help mitigate disputes.

This is untrue. Most HR, while the companies agent first, sees harassment as a huge liability and loves to be able to take action with sufficient documentation.

What about when the documentation is incomplete and it's one person's word against another's on a subjective body of messages and actions, and where the aggressor is often of a higher rank?

Sexual harassment often lacks sufficient documentation, and that's precisely why HR is usually not seen as an ally to the victim.

In some cases evidence is scarce, but in the story told in this article, there would seem to be a set of messages recorded in Slack, so it wouldn't just be one person's word against another. If the person being harassed wanted to make a really strong case, they would have sent a Slack reply saying "Don't ever send me this kind of message again!" to have a record of it.

This also makes it scary for HR, since if they do nothing and the victim sues, the Slack messages could turn up during discovery.

(I'm also guessing that this kind of behavior suggests that the aggressor is not of higher rank. Someone with real power over the victim probably wouldn't just stalk them on Slack, and any manager would be acutely aware that you don't want to say things like that in writing.)