Slack is also very popular with open source and hobbyist communities. People often seem to assume that it's used only at work, which I guess makes some amount of sense given that the vast majority of paying customers are probably workplaces. Slack would already be making money off me if they offered an attractively priced plan for open source and hobbyist communities and gave workspace owners an easy way to forward that monthly subscription fee onto the users. But at the moment I use Slack a lot and pay them nothing.

Then just leave yourself marked as offline all the time. I see many people that do that and face no kind of backlash for it.