It's in Drawable, not Layers
Tip: when you cannot find, there is a search box on top of that window, for this example I did input "flip" and the icon of Drawable is a "layer" on my GIMP's theme
Tip: when you cannot find, there is a search box on top of that window, for this example I did input "flip" and the icon of Drawable is a "layer" on my GIMP's theme
Patrice