I still see the Share button in the latest version on my Mac, but I think I saw some posts where it seems to be missing in the new M1 native version for Apple Silicon Macs. Are you using an Intel or Apple Silicon Mac, and which version of macOS?
If you are on an M1 Mac, the official Adobe help document for Photoshop for Apple Silicon says one of the currently missing features is “Share an image button / Quick Share.” Those missing features were available when running the non-M1 (Intel) version under Rosetta translation.
The Adobe help document doesn’t say whether those features are permanently gone, but some were restored between the beta and official releases, so more of them will probably come back in later updates. If you must use the Share button now, you could set Photoshop to open in Rosetta translation, but would lose the speed benefits of M1 optimization.
Same problem in using Photoshop 2021. The quick share button in upper right corner is non-functional. It's there, it just doesn't work. I restarted my MacBook Pro 15-inch (Mid-2015) running Big Sur. It worked just prior to the latest update of Photoshop 2021 but not since. Updated to Latest version of Big Sur as well (OS 11.3)