Earlier quoted context omitted.
Why? It seems to me the real thing open source projects ought to have is control over their data / autonomy - having some service control your forums and publish the code they use to run them doesn't seem like much of an advantage, if you can't apply patches to their system. For instance, while it helps Debian that their GitLab instance salsa.debian.org is open source, how does it help a non-Debian-affiliated project…
Having your OSS discussions on Slack is pretty terrible, it’s a closed source silo where things can’t be found via Google or even with the included search if you are above the limit. Every time I see an open source project asking people to “Join our Slack” it makes me sad. Besides, I’m in too many Slacks already. I really don’t need one Slack account for each project I want to follow.
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....)