If buildings are torn down, but still visible in current aerial imagery, it's recommended that rather than just delete them, you tag them with a [lifecycle][1] prefix. In your case you would probably want to use "demolished:".
Note that you'll end up with building buildings tagged "demolished:building=yes" rather than just "building=yes". This way they will no longer be rendered, but just act as a notification to other mappers.
[1]: https://wiki.openstreetmap.org/wiki/Lifecycle_prefix