Earlier quoted context omitted.
> Slack [...] just a good faith use of free software [...] Since when is Slack free software?
It sounds like you are conflating free software and Free software.
https://en.wikipedia.org/wiki/Free_software
In addition to not being free, Slack is also not software. It's a service.
Sure, there's some software involved in making that service tick (mostly proprietary), as well as some limited gratis access ("free tier") being offered, but that doesn't magically transform that service into "free software".