I have played World of Tanks casually since beta, and I like to keep a lot of tanks. In particular, I've collected handful of premium tanks via various promotions, which I rarely use, but which I like to keep nevertheless.
During my play time, a lot of tanks have been removed from the game for various reasons. Some times, these tanks are replaced by others, some times not.
I can see that a few of my (premium) tanks are no longer available in the game, but are still in my garage.
After seeing in-game chats and searching the forum (evidenced by the link in Chantola's answer), players seem to know ahead of time, which tanks will be removed from the game entirelly.
Where is this information available? Can I see it in-game somehow? Or is it simply hearsay and case-by-case?