It's also possible that a site may misleadingly return 451 codes to indicate that they've been censored when perhaps they haven't, maybe in order to inflame a d

It's also possible that a site may misleadingly return 451 codes to indicate that they've been censored when perhaps they haven't, maybe in order to inflame a discussion or some other political maneuvering. So, not only can you not trust that a missing 451 means no censorship, you also can't trust that a present 451 means there is censorship.