Why would you need metallic maps in a specular/smoothness workflow? It doesn't give you any benefits. You can just simply mask your metallic and dielectric materials in the specular map by using the correct pbr values. Thats why the specular/smoothness workflow exists to begin with. It's at minimum equal to the metallic/roughness workflow.

Maybe the metallic workflow is more convenient regarding the source files. That would be one benefit.
It is strange that other engines allow switch between workflow if it was without benefits.

Other engines don't allow to switch between both workflows. The specular input in a metallic workflow is just there to adjust the the reflection of dielectric materials in the 2-5% range. And when mask as metallic the specular information derives from the albedo map.