HN isn't really a bastion of media literacy or tech criticism. If you ever ask "does [some technology] affect [something qualitative] about [anything]", the response on hn is always going to be "technology isn't responsible, it's how the technology is used that is responsible!", asserting, over and over again, that technology is always neutral.
The idea that the mechanism of how people communicate affects what people communicate is a pretty foundational concept in media studies (a topic which is generally met with a hostile audience on HN). Slack almost certainly does play a role, but people who work in technology are incentivized to believe that technology does not affect people's behaviors, because that belief allows people who work in technology to be free of any and all qualitative or moral judgements on any grounds; the assertion that technology does not play a role is something that technology workers cling to because it absolves them of all guilt in all situations, and makes them, above all else, innocent in every situation. On the specific concept of a medium of communication affecting what is being communicated, McLuhan took these ideas to such an extreme that it's almost ludicrous, but he still had some pretty interesting observations worth thinking on, and his writing on this topic is some of the earlier work. This is generally the place where people first look, because much of the other work assumes you've understood McLuhan's work in advance. https://en.wikipedia.org/wiki/Understanding_Media