So Blender 2.7x or 2.8x added a feature called “Global Undo”. This appears to have hosed the ability to undo and redo palette modifications for me. It is really easy to trash your palette!
If this is affecting you, you can do one of two things:
1) Enable “Undo Legacy” from the Experimental tab in preferences.
OR
2) Disable “Global Undo” from the System tab in preferences.
Perhaps you can do both at once? But either or should work.