Quick answer: Godot 4 C# [Export] Resource property appearing greyed-out in the inspector? Read-only flag set by default for some property hints - explicit ReadOnly = false.
[Export] Texture2D Icon; inspector shows the slot but the drag-drop is disabled.
Inspector flags
Property attributes can include ReadOnly; remove or set explicitly.
Or use ExportTool
Some attributes mark editor-only writeable. Test with [Export] baseline.
Verify with simple test
Plain [Export] Resource MyResource; should work. If readonly, the issue is project-wide; investigate.
“Inspector behavior depends on attributes. Implicit flags surprise.”
If your inspector slots are unexpectedly readonly, the attribute combination is suspect. Document expected configs.