Yes, you can expect modern games to use any and all tricks available to squeeze performance. Imposters are an example of a LOD (Level of Detail) technique, which is standard in all game engines.
"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)
There are also tricks using a lower temporal resolution - for example giving fewer ticks per second to the logic or animation of entities further away/out of sight.