As long as there are traces - it can be kept `disused:amenity=restaurant` rather than `amenity=restaurant` etc. But once fully gone it should be removed - this

As long as there are traces - it can be kept `disused:amenity=restaurant` rather than `amenity=restaurant` etc. But once fully gone it should be removed - this way OSM data can be always verified/updated by visiting a given place, without being aware of now gone objects there. And keeps it objective - otherwise you would run into big discussion "where was this restaurant closed 20 years ago?" Or "how this castle was structured in year 1450" where you often have multiple competing reconstructions.