hi I am having the same problem,as this post: Unlinking materials from objects which is I need to get rid of unused materials but for 2.8. I have just recently converted to 2.8, and I had been able to remove them before but now unsure if these work now, and am still pretty confused by collections rather than layers and it seems materials maybe are stored there? anyways any clarification would be helpful?
actually i figured out how now by learning more about collections.
just remove from the collection area. things that are grayed out seem like are what are not used in a scene. and right click and you can delete them.
I am still having a lot of issues with 2.8 though. I think I might have found a bug or something in it, but will post about it somewhere else.