In macOS Tahoe Beta 2, Apple included a new option to add a background to the menu bar, making it possible to have a menu bar design that’s similar to the menu bar in macOS Sequoia.
As part of macOS Tahoe’s Liquid Glass design, Apple removed the background of the menu bar, so it blends into the wallpaper or background behind it. The icons of the menu bar are more free floating with the transparent look, though Apple does add a slight gradient to wallpapers to improve visibility.
Read more at MacRumors.com