If it was real what was the rationale behind the law? Was it merely personal preference or was there another reason?
Here's an older version of this question with one answer by u/kieslowskifan that agrees with what I've read about the issue. In short, it wasn't actually banned by law, but it was not welcome in the court theatre where these specific operas by Mozart were played. The reason for that was partly personal preference of the emperor, who was highly interested in art, and partly cost reduction.