In Unity URP (Universal Render Pipeline) we use the following standard:
Metallic in RGB + Smoothness in Alpha Channel
Smoothness is Roughness inverted, so it would be nice to have an option to invert the alpha channel before packing
Also, some Normal maps we download online have an inverted Green Channel... which comes from the difference between normal maps for DirectX and normal maps for OpenGL.
The same feature or channel flipping could be used to flip the green channel from these textures, and avoid having to resort to photoshop to flip the smoothness or the green channel from normal maps.