For the case of text, blur can be brute forced. If you redact, say, a credit card number with a blur, and I know what typeface the number would have been writt

For the case of text, blur can be brute forced. If you redact, say, a credit card number with a blur, and I know what typeface the number would have been written in, and have a reasonable guess as to your blur radius, it might not be infeasible to compare the blurred version of every possible credit card number. If you redact an email address with a blur, brute-forcing every possible email address will be harder. But if someone (say) leaks information to you, and you merely blur out their address, it's not infeasible that someone else could apply the same blur to a known suspect's email to verify whether it was them or not. Of course, with a large enough blur radius it's not an issue. Still, a non-zero amount of times, it's been done badly enough I've been able to mostly "reverse" a blur by just squinting and sitting back a few feet. Always redact text with solid blocks. I don't know how feasible this approach would be to human faces. I think Signal has blurred it such to make such an attack infeasible. I also don't think it's sufficient; if you don't want someone to be identified don't take photos of them, full stop and/or period. Take the photo at the top of the blog post. Who on that day, had that a backpack with that type of strap, a blue mask in exactly that shade of blue, that haircut, and that exact BLM t-shirt, in that place at that time of day? That could be sufficient information for a "fingerprint", though maybe not deanonymisation.