Again, Slack is just a tool. I have no problem with it, but maybe that's because I work in a company that has a healthy relationship with Slack, and I make generous use of its features to let me focus on what's relevant and interesting to me.
But, getting to the core of the matter:
> You can't mute the whole organization.
No, you can't. And you shouldn't. That's literally the whole point of organisations. If the particular mode of communication a given organisation uses isn't good enough, think about how to make it better instead of just brushing the whole thing off. We worked this out with phones, email and mobile phones. The discussion here is ripe with people saying "if it's important, they'll call me". Well, 20 years ago, mobile phones were given the Slack treatment because it was horrible that anyone could just call you.
Some bosses seem to reward underlings who respond to email at odd hours, yet nobody struggles to understand that it's toxic behaviour instead of writing diatribes about who we should stop using email altogether (although people did write those in the 90s). I heard of an organisation where the rumour was that some members of senior management would stalk employees' calendars to see if they were keeping busy. Good thing that organisation was able to end that kind of ridiculous Goodhart style management by ending the use of calendars!