Through bug report:
https://bugs.kde.org/show_bug.cgi?id=411593
I found out that wxgtk3 apps like audacity (or maybe even all gtk2 apps) don’t support global menu in Plasma because they require a package appmenu-gtk2 that doesn’t exist on Arch (but does on SUSE and maybe Debian).
Is there a chance to report it somewhere, so it would be added to Arch? It doesn’t even exist in AUR. Of course, it would be nice if Manjaro started packing it, but I won’t dare to hope.
This isn’t something vital, it’s just a nice touch and carefulness for details. This is astonishing that Arch lacks of some packages. I know that the technology is old, but the apps using it are still supported.
I know I could try to use rpm package and change it to arch format or install it manually, but I prefer to stay away from such manual changes. All should be dealt with pacman database and system updates.