I have never seen any credible documentary evidence that the tritone was "outlawed". That doesn't mean the evidence doesn't exist, just that I haven't seen it. The proper evidence of something being "outlawed" would, of course, have to consist of citations to legal sources such as canon law, civil law, proclamations by rulers, or records of court proceedings.

This linke shows that some mediveal theorists considered the tritone useful in special circumstances.

T.