In Catalina and earlier versions of macOS, certain keyboard shortcuts worked for dismissing alerts, but these same shortcuts no longer work for me in Big Sur.
You could and you can still cancel alert dialogs via esc and alerts with a recommended action (highlighted in blue) you can dismiss them with enter.
But you could also trigger an alert's action with, for example, ⌘ command + D, based on the title of the button (presumably).
No such keyboard shortcut I try works on the new Big Sur alerts so far. Is there a working keyboard shortcut here?