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".
Update: you've edited the parent since I responded to it. Your original comment was something like "it's not free, part of it is free"
Still no idea what you're moaning about tho...
That's news to me. Which part of Slack do you consider to be free software? Is that the part you think GDS is using?
[1] https://www.gnu.org/philosophy/free-sw.en.html
[2] https://dictionary.cambridge.org/dictionary/english/free-sof...
Slack can be a product, software, service. Depending on which angle you look at it.