It has appeared in Star Wars comic books and novels in recent years, and it has shown up in animated form plenty of times in Disney’s Star Wars Rebels, as well as in the corresponding Rebels tie-in lore.
On the contrary, the transporter has popped up in many different project, both canonical and non-canonical.
(Maybe another hint that Rebels will serve as connective tissue for Mandalorian’s universe.)
Now, just because The Mandalorian is the first live-action project to include the Imperial Troop Transport, that doesn’t mean the vehicle has only existed in toy form over the past four decades.