If you think that you can't do free software development without relying on all the capabilities of Slack, then the honest thing to do is to quit and spend the rest of your days just working on proprietary stuff. You obviously prefer the solution which has features over any other concern, so why would you waste your time having anything to do with free software?

Richard Stallman obviously used proprietary software. But not past the point when he had replace it, even imperfectly: "I began work on GNU Emacs in September 1984, and in early 1985 it was beginning to be usable. This enabled me to begin using Unix systems to do editing; having no interest in learning to use vi or ed, I had done my editing on other kinds of machines until then." [https://www.gnu.org/philosophy/fsfs/rms-essays.pdf] This tells us that Stallman used Unix. Well, why wouldn't he have; what other practical way was there to code anything? (One supposes he could have started from the bootloader on up, like the Unix guys before him.)

Stallman wouldn't use Unix today, since it has been replaced by free software, and he wouldn't use arguments like, well, such and such proprietary Unix has better memory management or faster I/O, so I will still use that.

If you're using things like Slack or Github without the slightest intent of working toward replacing them, yet using them for free software activities, then that is a comically conflicted position.