I saw a post from years ago that said that loot filter logic is tied into the engine and not moddable. Does anyone know if this still holds true? Modding aspects of the UI such as the Loot Filter seems to be much needed. Thanks!
You can move the buttons around 
What exactly do you want to do? Other than moving the button or changing the artwork in the filter UI, there’s not much you can change.
Look at Last Epoch’s loot filter, or Outlook’s email filter, or any simple rules-based filter worth anything; it can definitely be changed, and such a change would make any type of filtering possible, which is what I’d like to do. As a software developer, I don’t see how this can be hardwired into the engine as it’s essentially a callback function; you pass in the item, you get back a boolean as to whether it’s visible or not… it’s that simple, nothing I can see would tie it to the engine or internals. The game would then supply its default filter (callback), or a mod can supply its own. You would also need the ability to mod the UI, allowing for a filter screen/window that can be saved/loaded.