In cmake, Qt5GuiConfig.cmake will automatically include any file in the lib/cmake/Qt5Gui directory which is matching the Qt5Gui_*Plugin.cmake expression.

Qt5Gui_QIosOptionalPlugin_NSPhotoLibrary.cmake does not match so the target Qt5::QIosOptionalPlugin_NSPhotoLibrary is not available for use in iOS cmake environment.

Renaming the file Qt5Gui_QIosOptionalPlugin_NSPhotoLibraryPlugin.cmake would probably do the job.