I completely understand and respect that. It was not a complaint - but cognizance that you had a certain philosophy in mind.
Forget slack - look at hipchat which has far simpler privacy controls. The ability to create an organization (NOT linked to GitHub), per channel permissions is all that's needed. Hipchat makes this dead simple.
Lots of different people will have different requirements - some people will need Active Directory even, but essentially the only two things needed for enterprise messaging is permissions and search.
Forget slack - look at hipchat which has far simpler privacy controls. The ability to create an organization (NOT linked to GitHub), per channel permissions is all that's needed. Hipchat makes this dead simple.
Lots of different people will have different requirements - some people will need Active Directory even, but essentially the only two things needed for enterprise messaging is permissions and search.