I'm not who you were talking to, but while Slack was definitely intended for use by businesses, you can create a Slack "team" for free (or at least, that's what I remember). There are limits around free Slack (finite chat history) but it is possible.

> Also, have you ever tried a free software, decentralized alternative?

Which are you thinking of?

I've used Riot before, to see what Matrix was all about. The Riot client was kind of sluggish.

Anyways, I suspect the real reason people use things like Slack and Discord is not because it's the best tool for the job but because it's free for them to make a community. Typically, the person who administrates the server/team/community also has to pay a monthly subscription fee or pay to host it on a VPS, so it's only natural that people are flocking to a service which offers to let you do that for free.