Is it my imagination or did we have a transparent menubar before, and then it was decided against?
I'm not sure if this is a memory of an earlier version of MacOSX, or it might have been on Linux, in Gnome and/or Unity on Ubuntu, either way a few years ago now. This experiment has been tried before, and whichever platform it was on, they changed their mind and disabled it on the next release. Because it's OK as long as the background is co-operative (as with the default on Big Sur, of course), but with too many other backgrounds it just gets messy and indistinct if what's behind the menubar is too busy.