Adding ItemFlags to GuiItem

[imDMK]

Adding ItemFlags to GUI items is possible, but they don’t work in-game (tested in 1.21.3+, maybe even lower versions).

To make them work properly, you need to add empty AttributeModifiers to the ItemMeta, for example:
meta.setAttributeModifiers(MultimapBuilder.hashKeys().hashSetValues().build());
Without this, item flags are visually added but have no real effect in-game.

[Clexus]

That's "fixed" in 1.21.5+, just use tooltip display component, assuming you are using paper api