It can be found in several WC mappack maps, such as "sky_walk10.vmf" (with a dark skin, a secondary gun and an antenna), or "sniper_029.vmf" (with camouflage skin, no antenna and no secondary gun). It was cut along with the Conscripts, the APC being the only armored fighting vehicle left in the final version of the game.
Trivia
Four tank sounds originally used by the Merkava[1] are still in the Half-Life 2 sound files, in the folder "/sound/vehicles/", while six are in the playable Half-Life 2 Beta sound files, in the same folder.