When writing my chapter about words, I left out entirely any discussion of censorship, something ever-present in Italian opera throughout its history at least until the unification of Italy in 1870.
I think this must have been part of the attraction of the librettos of Metastasio, a native Roman and therefore someone familiar with the religious and political issues that aroused the attention of the censors. A composer could avoid the problem of censorship merely by choosing one of Metastasio's librettos to set.
Rossini began the movement away from Metastasio's librettos toward a more free-form libretto such as the French might use. The new style of opera was not what the church or the occupying armies had in mind, and they let their opinions be known. Verdi completely abandoned Stiffelio because he felt the censorship left nothing worth presenting.
This is a key issue in Divas and Scholars, which I won't repeat here. You'll have to read it for yourself.