https://wccftech.com/diablo-ii-resurrected-wont-feature-true-ultrawide-support-blizzard-confirms/

Diablo II: Resurrected is not going to feature true Ultrawide support on PC due to some issues that essentially break the game.

In a new post shared on the game's Official Forums, it has been confirmed that full 21:9 support will not be included in the final release of the game, as the Technical Alpha and the Beta have highlighted some issues related to AI and enemy pull. As such, the view in Ultrawide mode will be limited to 19:9 at launch, which the developer continuing to monitor the situation to find a different solution to these issues.