This should not be surprising to anyone. You're giving your data to a corporation who has every incentive to give it to your employer. The only real surprise here is that they weren't doing this earlier.
That's backwards. You're working for an employer who has every responsibility to monitor business communication. Slack can't "give" something they were never supposed to control in the first place and only have by accident of architecture. This fixes an old bug.
> You're working for an employer who has every responsibility to monitor business communication.
Setting aside for a second the unusual ethics being proposed here: it's trivial to bypass official channels if you want to say something you don't want your employer to see. It's fairly standard practice to not put anything you don't want someone else to read in text at all, but rather to pick up the phone--it's too easy for someone to copy/paste a Slack message or Forward an email. This can't be a responsibility of employers because very few employers can live up to that responsibility.
