I see this sentiment a lot and it strikes me that deep focus work simply isn't something that is needed or even wanted in most orgs. Just about every single org I've ever worked at prioritized a more extraverted work environment. In this regard, Slack is promoting the interests of the org and not harming them.
I personally have been focused on making positive changes with 15 solid minutes of effort. Because that's all I'm going to have before I'm interrupted. It's annoying and I would much prefer a quieter work environment, but that's clearly not what the org wants.
On the flip side I never have to feel bad about not being productive. I suspect most of the actual work on my team gets done at home after work hours, work is being increasingly treated like social hour.