No, Nintendo hasn't officially confirmed its release. But the official Zelda TOTK website of the game did say "Full version of game required to use DLC. Sold separately." back on 8th February 2023.
No, there are no existing or announced DLCs for the game as of July 17th, 2023.
Considering the game's success, there is a good chance Tears of the Kingdom gets a DLC sometime in the future. Following the pattern of BOTW, it should release in a few months around August 2023 or February 2024. This is just speculation.
Since TOTK covered the main story thoroughly, the DLC could either be focused on the stories of Mineru, Sonia, or Rauru. It could also be about the previous champions or what happened to the Divine Beasts. This is just speculation.