What is the keyboard shortcut for the Apple logo?

On an American keyboard layout, Shift+Option+K.

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Honestly, I found it one day when I was just fooling around with the various Option alternates. If you go to System Preferences → Keyboard → Input Sources tab and select "Show Input menu in menu bar," then use the Input menu to show the Keyboard Viewer, you can see exactly what is produced by the various Option key combinations.

The easy way to find this and other keys is to open the Keyboard Viewer and see it 🙂

Select the keyboard viewer. As you press the modifier keys in the actual keyboard on your mac, the keyboard viewer changes, to show what character corresponds to what key. Here it is when I hold down shift and option (as you see, they turn grey when pressed)