It probably helps we are over several timezones and each work irregular hours according to what's happening (kids pick up, teaching, sports..): we can't count on who is there at any time and consequently run the business asynchronously except for pre-planned meetings.
Open office doesn't mean that anyone can bother you at any time. It's important that companies set expectations about these things and enforce them to avoid that slippery slope.
Same goes for Slack. Once you expand beyond intra-team communication, it's important to set some expectations for what is and is not appropriate in Slack.
At the least happy point in my career, I had to be a member of over 100 Slack channels to get my job done and I received upwards of 100-200 @here and @channel notifications every day as people competed to stand above the noise. It was miserable. At the worst point, my time tracker was counting upwards of 4 hours per day spent in the Slack app.
Ironically, my efforts to push things to e-mails and scheduled calls was met with a lot of resistance from people who were afraid it would hamper flexibility and ruin asynchronous workflows. We also had absurd levels of office politics because people were treating each other as screen names instead of actual people working in other offices. I would take an open-office floorplan over that nonsense any day.
I can't recall where exactly but at one seminar I took, well before chat programs were ubiquitous, the presenter called out in-person interruptions. Just a co-worker dropping by randomly and starting a conversation. There are ways to deal with that, but those tricks don't work so well in a virtual world when you can have dozens of people simultaneously demanding your attention. (and TBH they stopped working very well with the move to open offices)
For me the biggest annoying thing about Slack is the calling feature. I don't like unscheduled calls. It also often starts making a racket when I'm in the middle of another call with different software. I might write a browser plugin sometime to disable Slack calls.
For everything else though I usually just close Slack when I'm heads down working and open it during periodic breaks to check for anything important.
The goal is to allow all of our team members to control what their own day/communication rules are, and make those shoulder taps opt-in, not out.
In the real world, if we ever return to it, I like having physical team rooms, the sort of thing you have naturally when it's an early-stage startup in a single space. When everybody knows each other and is focused on the same goal, it's not hard to work out good communication and space-use norms. But scaling that up into large open-plan offices can quickly turn maddening. It's the tragedy of the audio commons, and anybody can walk up and tap you on the shoulder.
For me Slack is the same problem, but worse in that it removes some of they physical signals and feedback mechanisms that work in an open-plan office.