How is that different from, say, IRC? How is it relevant that Slack is closed-source?

If you want to run an open-source project on Slack, run a logger that dumps things to a publicly Googleable archive - same as you would if you were running the project on IRC or on a mailing list or whatever.

If Slack says that running a logger is a violation of their terms, then that's a reason to avoid Slack in particular - but not with closed-source services in general. It's a problem with services run by someone else, open-source or not. And in fact, Freenode (which runs an open-source ircd) has a policy that if you log, you must say so clearly in the topic and you must respond to requests to remove portions of logs.

To be clear, I think that if a project says, out of idealism we will only use FLOSS forums, and we don't care about meaningful control over it, we just want to encourage other people to run FLOSS to promote the movement, that's totally reasonable. But if your concern about non-FLOSS platforms is practical and not philosophical, I'd argue what you actually care about is control over the platform.

> Besides, I’m in too many Slacks already.

I have this problem too, but it's not really about whether Slack is open-source or not, it's about the specific software. (Also, from experience, being in over a hundred channels in irssi is actually less manageable....)