From the AppKit Release Note for OS X v10.11 :
On 10.11, most NSOpenPanels accepted a project that was previously reserved only for applications that used iCloud Documents support, and this panel design was only used if iCloud Drive was included in System Preferences. This note will refer to the design as โiCloud Open Panel Design,โ although the design is no longer limited to iCloud document-enabled applications.
ICloud Open Panel Design doesnโt display an appearance, so it seems to be gone already. I do not think there is a way back.
If you want to display a message, description or title, you can use the message property.
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