disused: “features that are in a reasonable state of repair but which are currently unused”
In this context, disused would not fit since the landslide is most likely permanent and the road is not in a “reasonable state of repair”. I feel abandoned life cycle tag would be more appropriate here.
I would use demolished:highway= + note=destroyed by landslide in XYZ, still visible in aerial imagery such as Bing, please remove once no longer visible in standard imagery
(or with razed prefix or was … that is anyway for humans)