These are tanks that either can't be bought normally or are not yet fully in the game. They are announced on the Official Tanktastic VK group and WWII League VK group with development screenshots. Currently, the German Panzer series and the KV-1 are in the game, but the IS, BT-7, BT-2, and BMP-1 only exist as research tree icons. The T-28 has no icon in the game, but there is a screenshot of it.
The KV-1 and Panzer line can be fought against in Offline Mode, but cannot be bought in-game.