Comment 1 for bug 988997

About the "show desktop" shortcut, which is one of the most well known shortcuts, I think it would be better to keep the existing "Ctrl + Alt + D".

If the rational is that "Ctrl + Shift" is used to interact on a window while "Ctrl + Alt" lets interacting with workspaces, I would argue that while "Ctrl + Shift" interacts with the current window, this is not what "Show desktop" does.
It hides all the windows. Seen another way, it lets us view the workspace behind the current windows.
So I don't think that "Ctrl + Alt + D" to show the desktop would contradict the new "Ctrl + Shift" vs "Ctrl + Alt" distinction.