Imposters seems to be really old technique - see Trespasser 1998:

"The engine also used two other big tricks to lower the poly count: The most famous was "object caching": While rendering objects, the distant ones would be rendered to a cache and drawn as sprite on a quad. This trick originally resulted in mixed results since the mesh would remain a sprite too close from the player and changed from 2D to 3D in a very noticeable way as see in the two next images" (http://fabiensanglard.net/trespasser/index.php)