Soapstone, baby powder or talc which is the same material ground into a powder, is included in some flux coverings (E6010) as one component of the slag formers.
Since it is a mineral, it is not a hydrocarbon. As such, it isn't a hydrogen source when it decomposes in the welding arc. I would assume (a dangerous thing to do) it isn't hygroscopic to the degree where the absorbed moisture could be a significant source of diffusible hydrogen considering the quantity (of soapstone) deposited on the surface of the metal when a line is scribed is rather limited.
Best regards - Al