Xbox has announced that the upcoming Age of Empires III: Definitive Edition DLC announced last year has officially been canceled, a decision the devs say "was not made lightly".
In a post on Steam announcing the cancellation, the Age of Empires III: Definitive Edition development team says it canceled the DLC "after serious consideration", apologizing for not providing updates regarding said DLC since its announcement last February.
The DLC, which was revealed as part of the New Year, New Age event, came about because Xbox and the Age of Empires team wanted players to feel like "part of the franchise celebration", but that enthusiasm led to DLC being revealed "before it had been built".

Xbox and the Age of Empires III: Definitive Edition development teams say they now feel the DLC's announcement was "premature", hence the cancellation. They acknowledge this will come as a "huge disappointment" and pledge to continue maintaining the game's servers.
While the DLC was indeed announced last year, Xbox never officially revealed exactly what it would entail. All the studio released was a teaser showing World's Edge production director Earnest Yuen pointing to two flags: the Danish flag, and the flag of the old Polish-Lithuanian Commonwealth.
Since neither of these civilizations are available in Definitive Edition as it stands, it's logical to assume they'd have been introduced as part of the DLC, but since said DLC has been canned, it's a moot point now.
If you're wondering exactly what Age of Empires III: Definitive Edition is, it's a remastered version of the original Age of Empires III, complete with improved presentation and new content.
Revealed in full during Gamescom 2020, Age of Empires III: Definitive Edition packs in the base game and all DLC that had been released up to that point, as well as two new civilizations in the form of the Incans and the Swedes.
You can check Age of Empires III: Definitive Edition out right now on PC, and it's included as part of the PC Game Pass and Game Pass Ultimate tiers as well.