Unfortunately, a lot of startups - including successful ones - do use slack to mitigate the need for email, phone, and in-person communication. It baffles the mind but I think it speaks to the relative naivete and inexperience many managers at startups have when it comes to communication in the workplace.

Every time I have seen Slack as the primary communication vessel, I have seen a company that fails to ship on time and has a hard time with knowledge sharing. The medium is the message!