Just figured out how the NSStatusItem acquires its information of hight / appearance. If the „appearance“ key in the metrics dictionary is 0/1 it’s set to light- or dark-appearance. This is set on the NSStatusItem and can be acquired from the statusItem (or its button) via -effectiveAppearance. The metrics dictionary also includes the actual height of the menu bar (33px in case of a notch, 24 on a normal display).

As the appearance information depends on the system settings and wallpaper, it’s not as simple as querying for the system’s global appearance.