This seems like it completely misses the point of Slack: if someone is harassing you, you don’t report it to Slack Inc: you report it to your company’s HR department. And when your harasser tries to sue for wrongful dismissal, the company’s legal department has access to their DMs to present in court.

A product that has these features is the wrong tool for a large company; and similarly, if you need these features then Slack is the wrong tool for that job. Changing that is a social problem that would require restructuring the way we think about workplaces, not a technical problem that can be solved by getting Slack to add a few features.