It's a corporate communication tool. Harassment, etc. should be dealt with at the corporate (or legal, if required) level. It doesn't make sense to allow people to block their workmates. It's for work, not a dating app. Tinder needs and has blocking, Slack does not.
I don't understand why I can't mute certain people or group DMs when I can mute channels. Sometimes I need push notifications if I'm messaged from a specific person, but can't/don't want to be bothered by other specific people. There are ample productivity reasons for implementing this feature aside from the added bonus of tamping down on harassment.
If it's just a matter of turning off device-level notifications, then I agree (my office used MS Teams, not Slack). But actually blocking people so that you don't see their messages at all, or that they know they've been blocked? Not an appropriate feature for an internal comms tool IMO.
Corporate first, but there are very many users with the free version for smaller groups, which is what's the problem.