The Wikipedia page for TopoJSON (by Mike Bostock of D3.js fame) has been deleted

It was deleted for lack of importance. The only secondary source is a 2013 talk[1], which doesn't establish importance.

The link now goes to GeoJSON#TopoJSON, which is much better.

[1] http://vimeopro.com/openstreetmapus/state-of-the-map-us-2013...