They could still re-add it back. No one says they aren't allowed to return these rewards (except the Azeroth Choppers winning mount).
In cases like that they could just be vague enough that people would feel the need to resub, just in case they would get rid of it. As far as I remember it was also only ever clearly communicated that the 15th anniversary collectors edition mounts were available for a limited time only, but for the obsidian worldbreaker this wasn't as clearly communicated.