Ass-covering. It should've been obvious as soon as HR was mentioned that this isn't about helping employees, harassed or not.

Before this change, would the company have been liable for harassing DMs on Slack, an unaffiliated service where they previously were not able to read peoples DMs?

> an unaffiliated service

Unaffiliated? The company a customer that is paying Slack for a messaging service, that certainly isn't 'unaffiliated'.

Friction. Yes they could use PGP messages. But most people are idiots and use the systems that are available and simple. If somebody needs to think "I've got to go through these extra steps to be an asshole" then they might think twice in the first place.

No need for PGP. Using any other messenger that won't give private information out for no reason would be enough.

It increases the friction of harrassment

No it doesn't. It literally just removes liability from the corporation.

If you've ever dealt with toxic work relationships, you know that stopping harassment isn't going to come down to how you are allowed to pass notes.

Because you could just forbid employees from using such other form of communication at work.