To get screen sharing in Slack to properly work under Wayland, you need to explicitly tell Slack to use PipeWire for screen capturing. Copy Slack’s desktop file from /usr/share/applications/slack.desktop to ~/.local/share/applications/slack.desktop and modify the Exec line to look like:
Exec=/usr/bin/slack --enable-features=WebRTCPipeWireCapturer %U
Now restart Slack and screen sharing should work properly.