I don't think ALL reclaimed barn wood would be mold exposed and contaminated. There might be some that is perfectly fine.
But I would be careful with it. I've stayed in cabins with reclaimed wood beams and floors and the problematic spots for me were the spots where the wood was black in color, indicating previous water damage, I guess.
If I were going to try to use reclaimed wood, I would have to order it and put it in an enclosed room and then spend some time sitting in there with it to see if it bothered me.
And I would watch it like a hawk after it was installed to make sure it wasn't exposed to water and starting to decay. At least it would be visible, unlike wood that is used inside walls.
Best wishes with your project!