Nintendo has added a feature about Tetris 99 to their Japanese site, and it confirms the speculation on a retail release for Tetris 99. The retail release is indeed happening, and as we stated in our last feature, it's coming out Aug. 9th, 2019.
Here's where things get really interesting. This retail version is going to include the 'Complete Pack' DLC, which includes a local multiplayer mode for up to 8 players, and a mode where two players share one Joy-Con. With the 'Complete Pack' being involved included in the retail release, you would think that the DLC itself for the eShop version of Tetris 99 will come on that date at the latest, if not earlier.