Palettes can not be deleted after a custom palette has been hidden
Operating system is Mac OSX 10.14.6
You need a custom palette with a few elements in it to be visible in the list.
1: Hide a custom palette from the main list (don't delete it) - this can be done using the backspace button or by right clicking
2: After the custom palette is hidden, select another non-custom palette and press the Backspace button to hide it
3: Note that (almost always), the palette doesn't do anything and stays put
4: You can still delete the palette using the right-click or '...' menu
5: After following step 4 (above), the backspace button begins working again... until you try to hide another custom palette.