From OpenStreetMap Wiki
|Namespace for features that have been demolished, without any traces left.|
|Uses on taginfo|
|Used on these elements|
|Status: In use|
|Tools for this tag|
|This tag page describes a namespace rather than a simple tag.|
demolished: is a lifecycle prefix. It is used for features that have been demolished, without any traces left.
However, prior to 12/2014, it may have been used for things like ruins=* or when the feature is almost but not completely invisible.