Looking at the Lightning Design System's Combobox documentation, it has an example showing the styling for disabled list-items.
However, looking at the LWC lightning-combobox element, I can't see a way of creating disabled combobox list-items. I have tried adding disabled: true, or disabled: 'disabled' to the option objects for the list items, but it doesn't have any effect.
Is there a way to use the out-of-the box LWC lightning-combobox element to show some disabled list-items?
The lightning-combobox component does not implement most of the SLDS specification, much to my annoyance. I actually wrote my own combobox so I could implement the filter functionality (but I haven't implemented disabled items myself yet, as I don't have a use for it). You'll need to write your own component if you want some of those more advanced features.
There is a custom LWC based combobox component https://github.com/NPSINGH/LWC-Combobox, which inherit all the styling from the Lightning Combobox design blueprint https://www.lightningdesignsystem.com/components/combobox/. I'm not sure whether it supports the disabled property in options, but you can easily manipulate it since the source code is on Github.
