"Open source" was created specifically to avoid talking about freedom. If you want to talk about freedom, please say "Free Software".
In that case, we are talking about the freedom of the user to use software as they wish, to study it and modify it to suit their needs, and to share it with others. We're not talking about the freedom to go about their lives as they wish---that's a separate fundamental freedom, although it can certainly be restricted by nonfree software. Ideally, that software would also be Free, but that's their own choice.
But we should never encourage the use of proprietary software.
If you're talking about the "open source" development methodology, though, then the argument ceases to exist, because freedom is not important enough to discuss, and so those who adopt that perspective won't understand why it's bad to recommend Slack to someone, or even require it.
In that case, we are talking about the freedom of the user to use software as they wish, to study it and modify it to suit their needs, and to share it with others. We're not talking about the freedom to go about their lives as they wish---that's a separate fundamental freedom, although it can certainly be restricted by nonfree software. Ideally, that software would also be Free, but that's their own choice.
But we should never encourage the use of proprietary software.
If you're talking about the "open source" development methodology, though, then the argument ceases to exist, because freedom is not important enough to discuss, and so those who adopt that perspective won't understand why it's bad to recommend Slack to someone, or even require it.