You're either "engaged" and "available" (meaning you respond to messages in semi-real-time, pretty much nixing your ability to do deep work) or you feel like you're missing a lot of conversation that's going on behind your back, because you're heads down with Slack closed. Slack is good for (shitty) managers of remote teams, because you can see who's "in the office", and randomly ping people when your lack of planning skills means that you are working on something that you forgot to get an answer for in a deliberate way (instead of buzzing by their cube as you used to do)...but for developers I think it's a poor bargain.
There are better tools out there that encourage and support async workflows. Twist, Basecamp, Github discussions, even email with appropriate filters.