Well, appears intentional, but you can enable.
https://gitlab.gnome.org/GNOME/gimp/-/issues/7662
From the above, a settings.ini file in C:\Program Files\ Gimp 2.99\etc\gtk-3.0
https://gitlab.gnome.org/GNOME/gimp/-/issues/7662
From the above, a settings.ini file in C:\Program Files\ Gimp 2.99\etc\gtk-3.0
Code:
[Settings]
gtk-menu-images = true