We're currently in the middle of rolling 4.38.115 out to production now - you should be able to download and use it shortly! For additional context: Slack Desktop re-enabled the WebRTCPipeWireCapturer in version 4.38.115, after fixing two crashes that were previously preventing us enabling it by default. Sharing windows should now work as expected in Slack 4.38.115 and higher (no black screen). The engineers are still seeing intermittent issues with some machines where sharing your entire screen still shows as a black screen, and are working with Chromium upstream on a potential fix.
(source: https://forums.slackcommunity.com/s/question/0D7Hq0000099fAGKAY/detail?language=en_US)