last time I used it, Slack had a pretty bad design flaw. each organization has a #general channel that cannot be unsubscribed from (aka, every person in the company must be part of #general). it got very annoying when users spammed (@channel) the entire channel. many devs on my team sent feedback to Slack asking them for the ability to leave #general.

Their response was something like "we rely on everyone being in #general so that we have a single place to deliver broadcast messages"

This leads me to believe that their underlying code does not really have a concept of a user's 'organization', otherwise they could send based on that. Separate those concerns!