A very curious case is that huddles for me work in wayland (under sway), and don't work in X11 (under i3), for the exact same configuration otherwise. I have not investigated what exactly the browser sends.
Do the calls and huddles work in Chrome on Linux? Do they work in Firefox if you use a UA that doesn't mention Linux?
Yes and yes.
Thanks for testing. I'll ping Mozilla's web compat team about this Slack Linux problem. (I work at Mozilla.)
I was told Slack has some Linux-specific issues that they're working on with the Firefox team, such as some complications around screen sharing depending on the window manager (as mentioned in some other comments below about Wayland vs X11).
I couldn't use those through Safari on a Mac either. Nothing really tells you that it's unsupported. I could search or figure it out but I don't wear my programmer/software tester hat all the time.
Welp, better spend a few minutes downloading the same thing but in a Chrome sandbox.