[mrguyorama]

Your own links talks about how perceptual hashing hasn't been proven to be robust enough for this use case, and also introduces a new problem: Hash collisions, such that you can generate images that hash to the same perceptual hash as an illicit image.

[mikestew]

Robust or not, it's a little more nuanced than a file hash that goes out the window because a single pixel changed, as implied by GP.