If it was "outlawed", the two 14th century authors quoted by Ms. Schulter in the link posted above (which I duplicate here for convenience), Jacobus of Liege and Johannes Boen, hadn't heard of any such law. Jacobus of Liege says that the tritone is hard to sing and rare (he says the same of the dimished 5th). Johannes Boen says that the vertical tritone is a "consonance under special circumstances."

T.