The spec for MetalLookAndFeel.initClassDefaults(UIDefaults) says:
"...MetalLookAndFeel registers an entry for each of the classes
in the package javax.swing.plaf.metal that are named MetalXXXUI.
The string XXX is one of Swing's uiClassIDs. For the uiClassIDs
that do not have a class in metal, the corresponding class in
javax.swing.plaf.basic is used. For example, metal does not
have a class named "MetalColorChooserUI", as such,
javax.swing.plaf.basic.BasicColorChooserUI is used".

There is class MetalMenuBarUI, but the method populates given defaults table with the value
"javax.swing.plaf.basic.BasicMenuBarUI".