I don't see a keyboard shortcut for "Open Folder..." listed on the documentation page below, and after searching elsewhere for a while I didn't see anything else.
https://code.visualstudio.com/docs/customization/keybindings
I thought I would ask here before putting an issue on GitHub about it. (Also there doesn't seem to be a open / closed issue about this anywhere on the GitHub repo.)
There is indeed a shortcut for it, the command is workbench.action.files.openFolder
, and the default keybinding is Ctrl+K,Ctrl+O.
It's easy to find in the list of shortcuts (File → Preferences → Keyboard Shortcuts) if you search for "folder" or "openFolder" in the Default Keybindings.