It does not work because they intentionally disable pipewire support. You can binary patch the slack executable to enable wayland screen sharing https://askubuntu.com/a/1492207
Yeah screen sharing seems to be Wayland's biggest weakness. It doesn't work for me in Chrome, Firefox, OBS Studio, or VMWare Horizon (which explicitly says "Wayland is not supported").
Most of the others are just buggy, e.g. it "works" in Chrome but the window selector always shows the file browser icon at like 10x size so I can't select a window.
This is on RHEL 8, so maybe they've fixed things in the mean time.
I can't speak for Chrome nor VMWare, but Firefox and OBS have received "Wayland native" screen capturing functionality, through XDG portals and Pipewire, as far as I'm aware.
As a side-note, I've recently discovered a really cool project[0] that enables incredibly fast screen capture for OpenGL and Vulkan applications, mostly tailored to games. Tried it with a bunch of stuff and it works much better than both X's and Pipewire's screen grabbing. I can actually capture videos at my monitor's native refresh rate (144Hz).