Initially, there was a Feb 2026 release date announced for "additional story content", but it was released last week and the February date is no longer on any official sites or channels. IGDB currently has two pages:
https://www.igdb.com/games/pokemon-legends-z-a-mega-dimension
https://www.igdb.com/games/pokemon-legends-z-a-additional-story-content/
On the eShop, the Mega Dimension page is here: https://www.nintendo.com/us/store/products/pokemon-legends-z-a-mega-dimension-dlc-70070000030771-switch/
It says it includes two items, the Story Content and a small cosmetic item that was available at the base game's launch.
https://www.nintendo.com/us/store/products/holo-x-and-holo-y-apparel-items-70050000062490-switch/
https://www.nintendo.com/us/store/products/additional-story-content-70050000062491-switch/
As far as I can tell, it seems like the right move is to merge the Story Content page into the Mega Dimension page, there doesn't seem to be anything else coming out in February.
Buy Pokémon™ Legends: Z-A – Mega Dimension DLC and shop other great Nintendo products online at the official My Nintendo Store.
Buy Holo-X and Holo-Y Apparel Items and shop other great Nintendo products online at the official My Nintendo Store.