I don't see anything conductive to actual work that slack has but IRC lacks. the software industries and choices of tools such as chat clients are fashion-driven and most people will be hard pressed to provide a reason why they prefer slack or discord or whatever the newest shiny toy is other than "it's popular". They're popular, in turn, because their owners have marketing budgets, and decentralized, superior alternatives don't, because they are not commercial products. Just accept that any choice of tools or technologies is typically at least 95% irrational and dictated by marketing, not by rational reasons or calculations of risks and benefits. It used to be that companies would never risk third parties getting hold of confidential internal communications, nobody cares anymore though.