Yes, it fully works for me!

Terminal:

  system_profiler SPDisplaysDataType

Output:

macOS 26:

  Displays:
    LG HDR WQHD+:
      Resolution: 6400 x 2666
      UI Looks like: 3200 x 1333 @ 120.00Hz

macOS 27:

  Displays:
    LG HDR WQHD+:
      Resolution: 7680 x 3200
      UI Looks like: 3840 x 1600 @ 120.00Hz

144 Hz works as well.

I had to undo some of my previous configurations (0: disabled, 2: default)

  defaults -currentHost write -g AppleFontSmoothing -int 2

Apple lists ultrawide display support as a feature:

  Ultrawide display support. Now you can get higher resolutions on ultrawide displays, such as 5K at 120Hz. And your display arrangements stay exactly as you left them, for a seamless experience every time you plug in.

Copied from: https://www.apple.com/os/macos/

Previous discussion about macOS 26 Problem: https://news.ycombinator.com/item?id=47569502

I got a MacBook a few weeks ago, first Apple computer I have ever owned. I was so surprised when I plugged my 4K monitor into it, there didn't seem to be a way to run it at native resolution and all the text was aliased.