Arguments of type TGraphicsDrawEffect can optionally be used when drawing images out of an ImageList. The LCL performs a simple image manipulation to show the image in some specific state, e.g. grayed when the image represents a disabled item (gedDisabled). Or sometimes toolbar or menu icons are increased in brightness when the mouse hovers over them.
So, when you set the NodeEffect of a tree node to gdeDisabled it will be grayed out.
The attached demo cycles through all effects for the selected node by clicking on the button.
[EDIT]
Ah, you mentioned macOS... Could be that the node effects are not implemented for cocoa. I wrote and tested the demo project on Windows.
[EDIT2]
No - the NodeEffects are working in macOS. (But I noticed that the SelectedIndex always returns to -1...)