In Curse of Strahd, my cleric cast Banishment on the (possible spoiler):
Shadow Dragon
At the end of Banishment's duration, will the creature return? I don't know what plane it's native to. Our DM seemed unsure, himself. Our party has reason to believe that this creature is:
a silver dragon that was turned in to a shadow dragon by Strahd.
I don't have any confirmation, but I believe that the creature actually is:
Argynvost
Best Answer
In Barovia, the spell fails unless a specific condition is met
All that being said, in Barovia (emphasis mine):
... so the banishment spell has no effect until:
It is unclear if that event removes the restriction to planar travel or only permits inhabitants to leave through that specific method.
It's unclear for shadow dragons in general
Shadow dragons, according to their Monster Manual entry are:
In the former case, their plane of origin would be the Shadowfell, in which case, they would not return, remaining on the Shadowfell. Along a similar vein, a dragon born in most other planes who then was transformed, would also be permanently banished to that different plane.
On the other hand, a dragon born in Barovia who was transformed from some other effect would return (after being banished to a harmless demiplane).
The shadow dragon in Curse of Strahd:
If it is, as you suspect:
... then the dragon's origin plane is still unclear, as no mention of it is found in the module except that:
Without any further details of his origin, it will be up to the GM to decide if the dragon is native to Barovia or a different plane.