Or can metalness maps have values of grey in them? Also how can you bake a metalness map in blender? For this I would connect the nodes going into the metallic socket of the Principled BSDF to a viewer node and bake as emit, but if there are values inbetween 0-1 there will be grey areas on the map. When saving as a png I would use BW instead of RGB. What is correct?
Metalness maps can have grey in them, but in most cases I wouldn't recommend it as the transition usually doesn't look good, but do whatever you need to get the result you want. If there is a grey area in the map though, then it should be baked in and your approach with the emission node is the same as what I would do. You can save the image as BW to save on the file size. Hope that helps!